Updated filenames to snake case.

In Godot 3, GDQuest guidelines suggested using scenes and script files in pascal case. In Godot 4, this guidance changed and it's now recommended to use snake case for everything. I've renamed all filenames accordingly.

Updated delint script to check for snake case universally. This check is no longer restricted to just .gd and .tscn files, but also applies to .tres and other files.