Convert Learn GDScript App to Godot 4.6

[Razoric480]

Fixes #1187.

Uses the custom 4.6.1 build from https://github.com/Razoric480/custom-godot-builder which includes the GDScriptErrorChecker class (which takes the place of the 3.6 GDScriptParserWrapper).

Some notes on current State

• Converted project to godot 4. Can run through the first lesson, do its quizzes, and run the practice code.
• Fonts were pretty messed up. font_size is no longer part of the resource, and we have all these FontVariation resources whose sole job was to set the font size to various places. I've assigned font_size overrides to the theme and to the various TSCN files but they'll probably need a combing over for any I've missed, pending a rework of how those are assigned.
• Loooots of bugs and incomplete scripts, but it's otherwise in a "at least it runs and does what I expect it to" state.

Fun gotchas I ran into that gave me some trouble

• godot 3's right() function gave you the characters to the right of the index. godot 4's right() gives you X characters from the right side (which, to be fair, matches what left() does).
• dependence on yield() for stepped examples needed some hackery
• super._ready() and the like are no longer implicit and need to be called explicitly

[NathanLovato]

Made a couple more changes, notably removed the import cache folder from git tracking and updated the signal connections and the likes + corrected errors in the revealer script.