Fixes GDScript define nested dictionary and array as constants

[PastMoments]

Fixes #50285, note the regression was introduced in (#41983) (commit 3886a2f)

This issue seems to have been caused by the logic for checking const of elements being moved from reduce_array to const_fold_array (and reduce_dict to const_fold_dict) by the regression. Thus reduce_expression was not properly assigning const to these types.

This PR just adds extra checks in const_fold_array and const_fold_dict to handle nested arrays/dicts.

[akien-mga]

This seems to make a number of GDScript tests fail, and eventually crash: https://github.com/godotengine/godot/runs/5714947096?check_suite_focus=true

[PastMoments]

I've force pushed a new fix, and also added tests for the issue.
Tests seem to pass when I run locally (sorry I didn't do it properly the first time.)

[vnen]

Overall looks okay to me, despite my comment. This needs a rebase to remove the merge commit though (see https://docs.godotengine.org/en/stable/community/contributing/pr_workflow.html#the-interactive-rebase).

[PastMoments]

@vnen Resolved the comment and rebased.

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉