Remove freezed dependency

[stargazing-dino]

As much as I love and use freezed in my own projects, I don't think this is a necessary requirement for such a simple library. I can't really think of a better way to handle the sealed union nature of the AppState though and would love some suggestions.

This only seems like an issue for me now because freezed requires freezed_annotations and that depends on json_annotations which, thanks to nnbd, a lot of other libraries are not quite up to speed on requiring their v4.0.0 version. As a result, this will break dep resolution for some users.

Another option would to be to publish a stable 2.0.0 version that isn't nnbd and uses the older version. People could still depend on this package then without breaking things.

[stargazing-dino]

Nah, I'm going to close this. Feel free to comment to reopen