[WIP] Add --script-dump-ast command line argument to output an AST of a GDScript files

[DaughterOfZaun]

Resolves godotengine/godot-proposals#4958

[dalexeev]

Thanks for your contribution! I see it's still a work in progress, but I just want to say that I think you've chosen a suboptimal implementation. There's no need to add to_dictionary() method to every AST node, much less Object.

gdscript_parser.h is already too big. Besides, GDScript contributors use it as a reference for understanding the AST structure. We shouldn't clutter it with implementations or even just declarations of to_dictionary().

Instead, we can use an approach similar to TreePrinter and GDScriptEditorTranslationParserPlugin. But I wouldn't include it in gdscript_parser.{h,cpp}, and I'd factor out TreePrinter as well. Basically, we already have an AST dumper, but the output is not very parsable. We need JSON or something like that.

[DaughterOfZaun]

I definitely need help with this because I don't really understand what I'm doing.

• I'm not sure I'm calling the parser at the right time during initialization. Maybe it could be done earlier to make it faster.
• It would also be cool to (optionally) run the analyzer to infer types, but my attempts to make it work failed.
• It might be better to print the tree directly without converting to dictionaries, but I'm not sure if it's worth the trouble.

In any case, I consider this pull request a temporary measure, because I am convinced that the formatter and GDScript-to-CPP converter should exist in the editor - there is no need for AST export.