Allow optional : in extension

There's will probably be a tendency to put a `:` in a collective extension
by analogy to given and object, trait etc. We don't need to be picky about
this.

Author: odersky

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

@@ -3646,6 +3646,8 @@ object Parsers {
       val tparams = typeParamClauseOpt(ParamOwner.Def)
       val extParams = paramClause(0, prefix = true)
       val givenParamss = paramClauses(givenOnly = true)
+      in.observeColonEOL()
+      if (in.token == COLONEOL) in.nextToken()
       val methods =
         if isDefIntro(modifierTokens) then
           extMethod() :: Nil

tests/neg/i6900.scala

@@ -1,7 +1,7 @@
 object Test2 {
 
   // Works with extension method
-  extension [A](a: A)
+  extension [A](a: A):
     def foo[C]: C => A = _ => a   // error: extension method cannot have type parameters
 
   1.foo.foo

tests/pos/i6900.scala

@@ -21,7 +21,7 @@ object Test1 {
 object Test2 {
 
   // Works with extension method
-  extension [A, C](a: A)
+  extension [A, C](a: A):
     def foo: C => A = _ => a
 
   1.foo.foo