add gdscript support

Author: fengjiongmax

highlight/lib/languages/all.dart

@@ -61,6 +61,7 @@ import 'fsharp.dart';
 import 'gams.dart';
 import 'gauss.dart';
 import 'gcode.dart';
+import 'gdscript.dart';
 import 'gherkin.dart';
 import 'glsl.dart';
 import 'gml.dart';
@@ -258,6 +259,7 @@ final builtinLanguages = {
   'gams': gams,
   'gauss': gauss,
   'gcode': gcode,
+  'gdscript': gdscript,
   'gherkin': gherkin,
   'glsl': glsl,
   'gml': gml,