Fix not being able to open Godot 4 projects
I added a new class to read project.godot files, that checks the Godot version of the project and does the following:
- If it is a Godot 3 project, use the ConfigFile class to read the fields (as was being done previously).
- If it a Godot 4 project, use a set of regexes to obtain the desired fields.
This fixes #10.