Support -Xuse-experimental in the new MPP languageSettings { ... } DSL

Add the corresponding Gradle plugin DSL, consistency checks, and logic
for propagating these settings to the compiler during build.

Issue #KT-26840 In Progress

libraries/tools/kotlin-gradle-plugin-api/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/LanguageSettingsBuilder.kt

@@ -15,4 +15,8 @@ interface LanguageSettingsBuilder {
     fun enableLanguageFeature(name: String)
 
     val enabledLanguageFeatures: Set<String>
+
+    fun useExperimentalAnnotation(name: String)
+
+    val experimentalAnnotationsInUse: Set<String>
 }