Close #13417
Adds GDScript language support, autocomplete file, functionlist, default and dark themes. Fix #13329, close #13335
Fix #12517, close #12519
Fix #10353, close #10361
Use User Defined Language to have function list on Unit-tests. It could reduce regression happening while the code of boost regex or the related code is changed. And also make build system be aware of unit-test result. Close #10108