Add -source:3.9 and -source:3.9-migrations setting options

Author: WojciechMazur

compiler/src/dotty/tools/dotc/config/SourceVersion.scala

@@ -17,6 +17,7 @@ enum SourceVersion:
   case `3.6-migration`, `3.6`
   case `3.7-migration`, `3.7`
   case `3.8-migration`, `3.8`
+  case `3.9-migration`, `3.9`
   // Add 3.x-migration and 3.x here
   // !!! Keep in sync with scala.runtime.stdlibPatches.language !!!
   case `2.13`

library/src/scala/runtime/stdLibPatches/language.scala

@@ -364,6 +364,19 @@ object language:
   @compileTimeOnly("`3.8` can only be used at compile time in import statements")
   object `3.8`
 
+      /** Set source version to 3.9-migration.
+    *
+    * @see [[https://docs.scala-lang.org/scala3/guides/migration/compatibility-intro.html]]
+    */
+  @compileTimeOnly("`3.9-migration` can only be used at compile time in import statements")
+  object `3.9-migration`
+
+  /** Set source version to 3.9
+    *
+    * @see [[https://docs.scala-lang.org/scala3/guides/migration/compatibility-intro.html]]
+    */
+  @compileTimeOnly("`3.9` can only be used at compile time in import statements")
+  object `3.9`
 
   // !!! Keep in sync with dotty.tools.dotc.config.SourceVersion !!!
   // Also add tests in `tests/pos/source-import-3-x.scala` and `tests/pos/source-import-3-x-migration.scala`