Add support for loading unpacked ZIPs

[ithinkandicode]

This is a major update:

• Adds support for loading unpacked mods without ZIPs.
  • This lets you work on mods in the Godot editor, without needing to pack them into ZIPs for testing.
  • This also makes ZIP filenames arbitrary, which is helpful if you release ZIPs with versioned affixes
• Loads ZIP contents into the root
  • This handles importing files in your mod ZIP's .import folder, and matches the file system structure in the editor.
• Adds doc comments to every func.
• Optimizes the mod load process by disabling an expensive function that loops over files.

I've also updated the docs to recommend putting extender scripts in a dir named "extensions". This helps keep your mod folder tidy, as all extender scripts are separate from your mod's core files (eg ModMain.gd).

Breaking Changes

This update changes the way mod ZIPs are structured. I've updated the README.md to describe their new structure. In short, the zip's filename is now arbitrary, and the contents of the ZIP are loaded into the root. A mod's own files should be included in a folder called "mods-unpacked", and internal references to your mod's files should follow that path, eg:
res://mods-unpacked/Author-ModName/ModMain.gd

Here's an example ZIP:

Brotato-VersionString.zip

And here's how it looks when editing a Brotato project:

[otDan]

Looked thru it quickly and tested the provided build on discord, everything seems to be working correctly.

[ithinkandicode]

Closes #2
Closes #3
Closes #7

[ithinkandicode]

Compiled PCK for Brotato:
https://brotato.codemuffin.com/_files/Brotato--ModLoader-2.0.0.pck

ZIP of a few Brotato mods for testing (ContentLoader, Invasion, VersionString):
https://brotato.codemuffin.com/_files/mods.zip

[KANAjetzt]

Amazing stuff 👍
I will test it with my mods real quick and merge it 👍

[KANAjetzt]

Closes #2 Closes #3 Closes #7

We will have to keep #3 open until we find a solution for extending scripts in the editor.