refactor: ♻️ API: Move Config to a dedicated class

[ithinkandicode]

Moves all config JSON methods to a dedicated class, following the discussion in #153.

Like #156, this PR also uses a new directory called api.

I have tested this refactor and can confirm that it is working as before.

Other Changes

1. Adds a deprecation message to the old method.
   This is currently commented, pending feat: ✨ API: Add methods for deprecation + API Folder #156

2. Fixes an error: save_dictionary_to_file should have been save_dictionary_to_json_file.
   Aside from this, there are no other changes from the version currently on the development branch.

Breaking Changes

Deprecates ModLoader.get_mod_config. However, Brotato is the only game that contains this method, and its implementation ATOW is broken, such that loading a custom config JSON file does not work, so there aren't any mods that use it.

[ithinkandicode]

I have added a temporary ❌ to the PR name to indicate that another PR should be merged first: #156.

Once #156 is merged, the deprecation message in mod_loader.gd (here) can be uncommented.

[ithinkandicode]

There's an issue here as I've used ModLoader.logged_messages. I would have liked to have put it in config.gd, but I can't affect its member variables from these static funcs. However, it won't work at all if someone tries to use get_mod_config in their mod's _init, because it'll be before ModLoader exists. (Edit: Same with ModLoader.os_configs_path_override)

What would you guys suggest?

EDIT: @KANAjetzt Suggested the method outlined in #161, which is another autoload singleton to store data. This is a perfect fix IMO.

[Qubus0]

good.
We might want to consider refactoring it work as a resource like ModManifest, but that is something for later

[Qubus0]

Little thing, rest is good