patch 8.2.1270: Vim9: not skipping over function type declaration

Problem:    Vim9: not skipping over function type declaration with only a
            return type.
Solution:   Skip over the return type. (issue #6507)

src/testdir/test_vim9_func.vim

@@ -260,6 +260,16 @@ def Test_call_funcref()
   assert_equal(3, g:SomeFunc('abc'))
   assert_fails('NotAFunc()', 'E117:')
   assert_fails('g:NotAFunc()', 'E117:')
+
+  let lines =<< trim END
+    vim9script
+    def RetNumber(): number
+      return 123
+    enddef
+    let Funcref: func: number = function('RetNumber')
+    assert_equal(123, Funcref())
+  END
+  CheckScriptSuccess(lines)
 enddef
 
 let SomeFunc = function('len')