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>
+}