GDScript: minor issue

[masatake]

GDScript language reference:

• https://docs.godotengine.org/en/latest/tutorials/scripting/gdscript/gdscript_basics.html
• https://docs.godotengine.org/en/stable/development/file_formats/gdscript_grammar.html#doc-gdscript-grammar
• https://godotengine.org/article/gdscript-progress-report-new-gdscript-now-merged

The original discussion for designing the parser:

• #3194
• #3199

The critical issues:

• [ ] add ctags-lang-gdscript.7.rst.in.

The optional items:

• [ ] handling extends "file.gd".class
• [ ] parse @icon(...) after class_name
• [ ] remove unused code and share the code with python parser