Update Kotlin to 2.0 (#900)

- update Kotlin from 1.7.10 to 2.0.21
  - Kotlin 1.6 dependencies in Gradle lock files are expected because kotlinc,
    which is also used by some tests, internally uses some 1.6 dependencies
    for backwards compatibility reasons.
- update kotlinx-html and kotlinx-serialization
- adapt Kotlin code where necessary
- use Kotlin stdlib Path APIs where possible
- fix IntelliJ Kotlin inspection warnings
- reformat code with `./gradlew spotlessApply`
  - ktfmt adds lots of trailing commas
- Add workaround to fix IntelliJ "unresolved reference" errors

pkl-gradle/pkl-gradle.gradle.kts

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -53,7 +53,13 @@ sourceSets {
   test {
     // Remove Gradle distribution JARs from test compile classpath.
     // This prevents a conflict between Gradle's and Pkl's Kotlin versions.
-    compileClasspath = compileClasspath.filter { !(it.path.contains("dists")) }
+    //
+    // For some reason, IntelliJ import turns pklCommonsTest into a runtime dependency
+    // if `compileClasspath` is filtered, causing "unresolved reference" errors in IntelliJ.
+    // As a workaround, don't perform filtering for IntelliJ (import).
+    if (System.getProperty("idea.sync.active") == null) {
+      compileClasspath = compileClasspath.filter { !(it.path.contains("dists")) }
+    }
   }
 }
 
@@ -99,4 +105,6 @@ signing {
 // (Apparently, gradle-api.jar now contains metadata that causes kotlinc to enforce Gradle's Kotlin
 // version.)
 // A more robust solution would be to port plugin tests to Java.
-tasks.compileTestKotlin { kotlinOptions { freeCompilerArgs += "-Xskip-metadata-version-check" } }
+tasks.compileTestKotlin {
+  compilerOptions { freeCompilerArgs.add("-Xskip-metadata-version-check") }
+}