fix: redo state management with riverpod and models with freezed

[abhigyantrips]

This PR rebuilds the state management utilized by the previous implementation, with flutter_riverpod and completes the past models using freezed_annotations, which handles pretty much all the JSON function creation and linting automatically. Some of the changes include:

1. Shifting controllers completely to per-view directory, with their respective providers created with Riverpod.
2. Shifting models completely to per-view directory, utilizing Freezed for functionality and ease of use.
3. Adding custom_lint for both Riverpod and Freezed, with comments of the respective commands required to build them.
4. Removing 'repositories' completely and handling them through the controllers themselves (could do a separate services/ directory, but don't think that is required as of now).
5. [Extra] Change the GNav from google_nav_bar with the normal NavigationBar from Material 3, for the consistency and accessibility. Gonna add a backdrop filter to it for flair, still experimenting with that.

[abhigyantrips]

Also yeah this involuntarily closes #14 lol

[abhigyantrips]

Was having an issue with the ContributorsController before, turns out the GitHub API returns the fields in snake_case while our properties are in camelCase. Thank the lords that JsonSerializable exists and all it required was a change in the fieldRename property.

The contributors' view is fixed, still need to finish up the current implementation of EventsView, but that might be a part of another PR as we need to completely redo the current model and fetching (since we're replacing Sanity with our own API).

Hence, merging changes for now. :)

[abhigyantrips]

This is what the app looks like as of now.

Record_2024-03-09-13-35-18.mp4