Convert build script to Kotlin

- Add a new Node plugin. This automatically installs npm dependencies
   and provides a "NpxExecToDir" to dir task. This allows us to make the
   doc website task dependencies a little nicer, by simply chaining
   tasks together, rather than doing dependsOn + `input.files(the other
   task output)`.

 - Switch over to CurseForgeGradle from CurseGradle. The latter is
   super clunky to use in non-Groovy languages.

 - Copy our Modrinth description body to our repo, and add support for
   syncing it. We'll still have to do CF manually I think.

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

@@ -0,0 +1,60 @@
+package cc.tweaked.gradle
+
+import org.gradle.api.DefaultTask
+import org.gradle.api.Plugin
+import org.gradle.api.Project
+import org.gradle.api.file.Directory
+import org.gradle.api.file.DirectoryProperty
+import org.gradle.api.provider.Provider
+import org.gradle.api.tasks.*
+import java.io.File
+
+class NodePlugin : Plugin<Project> {
+    override fun apply(project: Project) {
+        val extension = project.extensions.create("node", NodeExtension::class.java)
+        project.tasks.register(NpmInstall.TASK_NAME, NpmInstall::class.java) {
+            projectRoot.convention(extension.projectRoot)
+        }
+    }
+}
+
+abstract class NodeExtension(project: Project) {
+    /** The directory containing `package-lock.json` and `node_modules/`. */
+    abstract val projectRoot: DirectoryProperty
+
+    init {
+        projectRoot.convention(project.layout.projectDirectory)
+    }
+}
+
+/** Installs node modules as dependencies. */
+abstract class NpmInstall : DefaultTask() {
+    @get:Internal
+    abstract val projectRoot: DirectoryProperty
+
+    @get:InputFile
+    @get:PathSensitive(PathSensitivity.NONE)
+    val packageLock: Provider<File> = projectRoot.file("package-lock.json").map { it.asFile }
+
+    @get:OutputDirectory
+    val nodeModules: Provider<Directory> = projectRoot.dir("node_modules")
+
+    @TaskAction
+    fun install() {
+        project.exec {
+            commandLine("npm", "ci")
+            workingDir = projectRoot.get().asFile
+        }
+    }
+
+    companion object {
+        internal const val TASK_NAME: String = "npmInstall"
+    }
+}
+
+abstract class NpxExecToDir : ExecToDir() {
+    init {
+        dependsOn(NpmInstall.TASK_NAME)
+        executable = "npx"
+    }
+}