Add explicit @experimental to inner experimental classes

MiMa seems to check if the definition is experimental but it is missing
the check if the owner is experimental.

library/src/scala/annotation/MainAnnotation.scala

@@ -91,6 +91,7 @@ object MainAnnotation:
    *  @param documentation The documentation of the main method without the `@param` documentation (see Parameter.documentaion)
    *  @param parameters Information about the parameters of the main method
    */
+  @experimental // MiMa does not check scope inherited @experimental
   final class Info(
     val name: String,
     val documentation: String,
@@ -111,6 +112,7 @@ object MainAnnotation:
    *  @param documentation The documentation of the parameter (from `@param` documentation in the main method)
    *  @param annotations The annotations of the parameter that extend `ParameterAnnotation`
    */
+  @experimental // MiMa does not check scope inherited @experimental
   final class Parameter(
     val name: String,
     val typeName: String,
@@ -121,6 +123,7 @@ object MainAnnotation:
   )
 
   /** Marker trait for annotations that will be included in the Parameter annotations. */
+  @experimental // MiMa does not check scope inherited @experimental
   trait ParameterAnnotation extends StaticAnnotation
 
 end MainAnnotation

library/src/scala/annotation/newMain.scala

@@ -217,7 +217,7 @@ final class newMain extends MainAnnotation[FromString, Any]:
         hasParseErrors = true
         () => throw new AssertionError("trying to get invalid argument")
 
-
+  @experimental // MiMa does not check scope inherited @experimental
   private object Help:
 
     /** The name of the special argument to display the method's help.
@@ -321,6 +321,7 @@ final class newMain extends MainAnnotation[FromString, Any]:
 
   end Help
 
+  @experimental // MiMa does not check scope inherited @experimental
   private class Names(info: Info):
 
     checkNames()

tests/run-custom-args/tasty-inspector/stdlibExperimentalDefinitions.scala

@@ -31,13 +31,20 @@ val experimentalDefinitionInLibrary = Set(
   // Should we have better support for main annotation macros?
   "scala.annotation.MainAnnotation",
   "scala.annotation.MainAnnotation$",
+  "scala.annotation.MainAnnotation$.Info",
+  "scala.annotation.MainAnnotation$.Parameter",
+  "scala.annotation.MainAnnotation$.ParameterAnnotation",
+
 
   //// New feature: prototype of new version of @main
   // This will never be stabilized. When it is ready it should replace the old @main annotation (requires scala.annotation.MainAnnotation).
   // Needs user feedback.
   "scala.annotation.newMain",
   "scala.annotation.newMain$",
   "scala.annotation.newMain$.alias",
+  "scala.annotation.newMain.Help",
+  "scala.annotation.newMain.Help$",
+  "scala.annotation.newMain.Names",
 
   //// New APIs: Mirror
   // Can be stabilized in 3.3.0 or later.