Add working ResourceFormatLoaderGDScript::get_dependencies() implementation

[nfranck-toonboom]

Remove stub code from GDScripts' get_dependencies implementation, add working code for finding dependencies from parent classes and constant declarations

• Fixes "Export Selected Scenes" ignores "class_name" dependencies #51717.
• Fixes ResourceFormatLoaderGDScript::get_dependencies always returns an empty list #90643.

Bugsquad edit: Linked issue.

[adamscott]

Remove stub code from GDScripts' get_dependencies implementation

For some context, this isn't related to GDScript::get_dependencies() (cyclic dependencies checks). This PR is instead about ResourceFormatLoaderGDScript::get_dependencies() which is called by ResourceFormatLoader.

[dalexeev]

See also:

• ResourceFormatLoaderGDScript::get_dependencies always returns an empty list #90643
• GDScript: Implement get_dependencies() (reverted) #90860
• Revert "GDScript: Implement get_dependencies()" #91908

This may be more complicated than it seems. Let's move this to the next milestone. CC @vnen @adamscott