Allow end given as an end marker

Allow `end given` as an end marker for anonymous givens.
These could not have an end marker before.

Based on scala#7557.

Author: odersky

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -3412,7 +3412,7 @@ object Parsers {
         else
           (EmptyTermName, isIdent(nme.extension))
 
-      val gdef = indentRegion(name) {
+      val gdef = indentRegion(if name.isEmpty then GIVEN else name) {
         if isExtension then
           if (in.token == COLON) in.nextToken()
           assert(ident() == nme.extension)

compiler/src/dotty/tools/dotc/parsing/Scanners.scala

@@ -345,7 +345,7 @@ object Scanners {
 
         lookahead.token match {
           case IDENTIFIER | BACKQUOTED_IDENT => handle(lookahead.name)
-          case IF | WHILE | FOR | MATCH | TRY | NEW => handle(lookahead.token)
+          case IF | WHILE | FOR | MATCH | TRY | NEW | GIVEN => handle(lookahead.token)
           case _ =>
         }
       }

tests/pos/end-given.scala

@@ -0,0 +1,3 @@
+given Conversion[Int, String] with
+  def apply(x: Int) = ""
+end given