Use a Gradle plugin to download illuaminate

Previously illumainate required manual users to manually download it and
place it in ./bin/. This is both inconvenient for the user, and makes it
hard to ensure people are running the "right" version.

We now provide a small Gradle plugin which registers illuaminate as a
ependency, downloading the appropriate (now versioned!) file. This also
theoretically supports Macs, though I don't have access to one to test
this.

This enables the following changes:

 - The Lua lint script has been converted to a Gradle task (./gradle
   lintLua).

 - illuaminateDocs now uses a task definition with an explicit output
   directory. We can now consume this output as an input to another
   task, and get a task dependency implicitly.

 - Move the pre-commit config into the root of the tree. We can now use
   the default GitHub action to run our hooks.

 - Simplify CONTRIBUTING.md a little bit. Hopefully it's less
   intimidating now.

buildSrc/src/main/kotlin/cc/tweaked/gradle/ExecTasks.kt

@@ -0,0 +1,11 @@
+package cc.tweaked.gradle
+
+import org.gradle.api.provider.Property
+import org.gradle.api.tasks.AbstractExecTask
+import org.gradle.api.tasks.OutputDirectory
+import java.io.File
+
+abstract class ExecToDir : AbstractExecTask<ExecToDir>(ExecToDir::class.java) {
+    @get:OutputDirectory
+    abstract val output: Property<File>
+}

buildSrc/src/main/kotlin/cc/tweaked/gradle/Illuaminate.kt

@@ -0,0 +1,121 @@
+package cc.tweaked.gradle
+
+import org.gradle.api.DefaultTask
+import org.gradle.api.Plugin
+import org.gradle.api.Project
+import org.gradle.api.Task
+import org.gradle.api.artifacts.Dependency
+import org.gradle.api.provider.Property
+import org.gradle.api.provider.Provider
+import org.gradle.api.tasks.AbstractExecTask
+import org.gradle.api.tasks.Input
+import org.gradle.api.tasks.TaskAction
+import java.io.File
+
+abstract class IlluaminateExtension {
+    /** The version of illuaminate to use. */
+    abstract val version: Property<String>
+
+    /** The path to illuaminate. If not given, illuaminate will be downloaded automatically. */
+    abstract val file: Property<File>
+}
+
+class IlluaminatePlugin : Plugin<Project> {
+    override fun apply(project: Project) {
+        val extension = project.extensions.create("illuaminate", IlluaminateExtension::class.java)
+        extension.file.convention(setupDependency(project, extension.version))
+
+        project.tasks.register(SetupIlluaminate.NAME, SetupIlluaminate::class.java) {
+            file.set(extension.file.map { it.absolutePath })
+        }
+    }
+
+    /** Set up a repository for illuaminate and download our binary from it. */
+    private fun setupDependency(project: Project, version: Provider<String>): Provider<File> {
+        project.repositories.ivy {
+            name = "Illuaminate"
+            setUrl("https://squiddev.cc/illuaminate/bin/")
+            patternLayout {
+                artifact("[revision]/[artifact]-[ext]")
+            }
+            metadataSources {
+                artifact()
+            }
+            content {
+                includeModule("cc.squiddev", "illuaminate")
+            }
+        }
+
+        return version.map {
+            val dep = illuaminateArtifact(project, it)
+            val configuration = project.configurations.detachedConfiguration(dep)
+            configuration.isTransitive = false
+            configuration.resolve().single()
+        }
+    }
+
+    /** Define a dependency for illuaminate from a version number and the current operating system. */
+    private fun illuaminateArtifact(project: Project, version: String): Dependency {
+        val osName = System.getProperty("os.name").toLowerCase()
+        val (os, suffix) = when {
+            osName.contains("windows") -> Pair("windows", ".exe")
+            osName.contains("mac os") || osName.contains("darwin") -> Pair("macos", "")
+            osName.contains("linux") -> Pair("linux", "")
+            else -> error("Unsupported OS $osName for illuaminate")
+        }
+
+        val osArch = System.getProperty("os.arch").toLowerCase()
+        val arch = when {
+            osArch == "arm" || osArch.startsWith("aarch") -> error("Unsupported architecture '$osArch' for illuaminate")
+            osArch.contains("64") -> "x86_64"
+            else -> error("Unsupported architecture $osArch for illuaminate")
+        }
+
+        return project.dependencies.create(
+            mapOf(
+                "group" to "cc.squiddev",
+                "name" to "illuaminate",
+                "version" to version,
+                "ext" to "$os-$arch$suffix",
+            ),
+        )
+    }
+}
+
+private val Task.illuaminatePath: String? // "?" needed to avoid overload ambiguity in setExecutable below.
+    get() = project.extensions.getByType(IlluaminateExtension::class.java).file.get().absolutePath
+
+/** Prepares illuaminate for being run. This simply requests the dependency and then marks it as executable. */
+abstract class SetupIlluaminate : DefaultTask() {
+    @get:Input
+    abstract val file: Property<String>
+
+    @TaskAction
+    fun setExecutable() {
+        val file = File(this.file.get())
+        if (file.canExecute()) {
+            didWork = false
+            return
+        }
+
+        file.setExecutable(true)
+    }
+
+    companion object {
+        const val NAME: String = "setupIlluaminate"
+    }
+}
+
+abstract class IlluaminateExec : AbstractExecTask<IlluaminateExec>(IlluaminateExec::class.java) {
+    init {
+        dependsOn(SetupIlluaminate.NAME)
+        executable = illuaminatePath
+    }
+}
+
+abstract class IlluaminateExecToDir : ExecToDir() {
+    init {
+        dependsOn(SetupIlluaminate.NAME)
+        executable = illuaminatePath
+    }
+}