build: convert Gradle build scripts from Groovy to Kotlin DSL

Converts every .gradle file in the repo (root, build-logic's precompiled
plugins, desktop/engine/modules/templates) to .gradle.kts.

Notable non-mechanical changes required along the way:
- build-logic switches from the groovy-gradle-plugin to kotlin-dsl plugin;
  its precompiled scripts move from src/main/groovy to src/main/kotlin.
- GestaltExtension is extracted into its own GestaltExtension.kt: Kotlin
  script files don't share top-level type declarations across files the
  way Groovy scripts implicitly do.
- Several scripts (destination-sol-jre, destination-sol-module,
  engine/build.gradle.kts, desktop/build.gradle.kts) look up the
  base/idea/eclipse extensions explicitly via the<T>()/configure<T>()
  instead of the generated type-safe accessors: those plugins are applied
  transitively through another precompiled plugin rather than directly in
  the consuming script's own plugins{} block, and Gradle's accessor
  generation doesn't chase through that chain.
- destination-sol-ide.gradle.kts's IDEA XML patching (compiler.xml,
  misc.xml, checkstyle-idea.xml) is ported off Groovy's Node '.@attr'
  sugar onto groovy.util.Node's plain Java API via small child()/attr()
  helpers. Verified by actually running `gradlew idea` - not just
  compiling - since this is exactly the kind of dynamically-typed code a
  naive port could silently get wrong.
- gestalt-module.gradle.kts's module.json/module.txt parsing moves from
  Groovy's JsonSlurper to Gson (added as a build-logic dependency), fully
  typed.
- The publishing blocks in gestalt-module, terasology-publish-common and
  destination-sol-module all configure the same, project-name-keyed
  MavenPublication. Groovy's "$project.name"(MavenPublication){} sugar
  silently reconfigures an existing publication; Kotlin's create<>() does
  not, it throws. Switched all three to maybeCreate().
- Dropped ipr.withXml{}/workspace.iws.withXml{} calls to
  ideaActivateCheckstyle/Copyright/Annotations/Git/Gradle,
  ideaMakeAutomatically and ideaRunConfig in the root build script: none
  of the 7 functions have existed since config/gradle/ide.gradle was
  deleted in 6819386 (Dec 2022). Groovy only fails on missing methods at
  runtime, when `gradle idea`/`ipr`/`iws` actually executes, which
  nothing does - so this has been silently dead for ~3 years. Kept the
  one working sibling line (wildcards.remove) alongside it.
- .gitignore's !modules/subprojects.gradle and !libs/subprojects.gradle
  negations are updated to the new .kts filenames, otherwise the broader
  modules/*/libs/* ignore rules swallow the renamed files.

Verified: gradlew help configures the full project (root, build-logic,
desktop, engine, templates, and the auto-templated modules:core).
:engine:compileJava and :desktop:compileJava both build clean. gradlew
idea and :desktop:eclipse both run end-to-end, exercising the hand-ported
XML logic at runtime rather than just at compile time.

No behavior change intended anywhere in this diff; the findbugs ->
error-prone swap is a separate, follow-up PR.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: soloturn

.gitignore

@@ -11,10 +11,11 @@ gwt
 steam
 modules/*
 modules/**/build.gradle
+modules/**/build.gradle.kts
 !modules/core
-!modules/subprojects.gradle
+!modules/subprojects.gradle.kts
 libs/*
-!libs/subprojects.gradle
+!libs/subprojects.gradle.kts
 
 ## GWT
 war/

build-logic/build.gradle

@@ -0,0 +1,15 @@
+plugins {
+    `kotlin-dsl`
+}
+
+repositories {
+    gradlePluginPortal()
+}
+
+dependencies {
+    implementation("com.google.code.gson:gson:2.10.1")
+    implementation("com.github.spotbugs.snom:spotbugs-gradle-plugin:5.2.3")
+    implementation("ru.vyarus:gradle-animalsniffer-plugin:2.0.1")
+    implementation("de.undercouch:gradle-download-task:5.7.0")
+    implementation("gradle.plugin.org.jetbrains.gradle.plugin.idea-ext:gradle-idea-ext:1.4.1")
+}