Overhaul Mod Menu

[SmushyTaco]

This makes the code readable, updates dependencies, uses JSpecify for nullability annotations, and fixes scrolling.

Closes #2.
Closes #3.

[SmushyTaco]

Added some more finishing touches. The mods folder is opened without the use of LWJGL 2 or the legacy File API. It's opened in a more reliable manner, same with URLs. URL buttons also aren't meant to be clickable if there's no URL, but if mod authors incorrectly set an empty string as a URL, it'll be clickable even though it leads nowhere. The changes I've made have fixed this too. So scrolling is fixed, the codebase is more readable, mixins are improved, etc.

[SmushyTaco]

Now this also closes #2.

[perdakovich]

amazing