feat: add spell checker for English

[iagorrr]

feat: add spell checker for English

    - The plugin `spellwarn` was added, and set to support English.
    - The flake.nix and flake.lock was also update to include the new plugin.
    - A file size limit as added to avoid lag when dealing with large files
    (larger than 80 lines)

[SystematicError]

Thanks for the PR! I haven't really tried out spellchecking code, so I'm not too sure if this is a feature that should be there by default, I'll test it out and see how it goes.

PS: I've added some documentation regarding custom user configuration in the README, you can define your own plugins and configs through that.

[iagorrr]

... so I'm not too sure if this is a feature that should be there by default, I'll test it out and see how it goes.

The built-in Neovim spell checker works very well; however, I find that the SpellWarn plugin becomes too laggy with larger files or too many errors, which makes for a poor user experience. Maybe only the built-in should be default, but i'll put some extra time to check if i made anything wrong.

PS: Thanks for the documentation :))

[iagorrr]

... but i'll put some extra time to check if i made anything wrong.

Still, I couldn’t reach a usable state — the plugin made everything feel laggy.
Meanwhile, the built-in spell checker works exceptionally well out of the box; perhaps it should be enabled by default?

Would literally be just this:

vim.opt.spell = true
vim.opt.spelllang = { "en_us" }