Default version TODOs

[elenakrittik]

• Add a constant Godot path that symlinks to whichever the current default is. (previously: feat(gdtk-cli, gdtk-gvm): Implement global godot symlinks for default versions. #88 )
• Add a gdtk godot default command that works similarly to rustup's rustup default.

[elenakrittik]

#1 undone in #106 because i felt like it it was assigning the default version the wrong way