Publish ContractsDsl

- Mark @ContractsDsl as @Experimental
- Move Contracts DSL out from 'internal' package
- Change visibility of ContractsDsl from 'internal' to 'public'

^KT-25274 Fixed
^KT-25495 Fixed

compiler/util/src/org/jetbrains/kotlin/config/LanguageVersionSettings.kt

@@ -74,6 +74,7 @@ enum class LanguageFeature(
     ReadDeserializedContracts(KOTLIN_1_3),
     UseReturnsEffect(KOTLIN_1_3),
     UseCallsInPlaceEffect(KOTLIN_1_3),
+    AllowContractsForCustomFunctions(KOTLIN_1_3),
     VariableDeclarationInWhenSubject(KOTLIN_1_3),
     ProhibitLocalAnnotations(KOTLIN_1_3, kind = BUG_FIX),
     ProhibitSmartcastsOnLocalDelegatedProperty(KOTLIN_1_3, kind = BUG_FIX),
@@ -100,8 +101,6 @@ enum class LanguageFeature(
         State.ENABLED_WITH_WARNING
     ),
 
-    AllowContractsForCustomFunctions(sinceVersion = null, defaultState = State.DISABLED, kind = UNSTABLE_FEATURE),
-
     MultiPlatformProjects(sinceVersion = null, defaultState = State.DISABLED),
 
     NewInference(sinceVersion = KOTLIN_1_3, defaultState = State.DISABLED),