Relocate our existing web code to subdirectories

- Move the frontend code into src/frontend
 - Move our custom element SSR system into src/htmlTransform.

This is mostly in prep for merging in copy-cat's core, as that's a whole
bunch of extra code.

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

@@ -5,6 +5,7 @@
 package cc.tweaked.gradle
 
 import org.gradle.api.artifacts.dsl.DependencyHandler
+import org.gradle.api.file.FileSystemLocationProperty
 import org.gradle.api.provider.Property
 import org.gradle.api.provider.Provider
 import org.gradle.api.tasks.JavaExec
@@ -124,3 +125,6 @@ class CloseScope : AutoCloseable {
 
 /** Proxy method to avoid overload ambiguity. */
 fun <T> Property<T>.setProvider(provider: Provider<out T>) = set(provider)
+
+/** Short-cut method to get the absolute path of a [FileSystemLocationProperty]. */
+fun FileSystemLocationProperty<*>.getAbsolutePath(): String = get().asFile.absolutePath