Add types to @stylex/nextjs-plugin package

[lukewhitehouse]

Is your feature request related to a problem? Please describe.
The current implementation of the NextJS plugin doesn't use Flow as other packages like @stylexjs/babel-plugin, so it doesn't include TypeScript or Flow type definitions.

This causes the following error when instantiating the module within a consuming applications next.config.js if you have it included in your tsconfig:

Could not find a declaration file for module '@stylexjs/nextjs-plugin'.

Describe a solution you'd like
This was recently added to the Rollup plugin in #60, closing #46

Describe alternatives you've considered
This will follow the same solution as other packages, so I wouldn't recommend an alternative.

Additional context
N/A.

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Open to giving this a shot, I've not used Flow before but I should be able to follow the Rollup PR.

[lukewhitehouse]

I've made a start with the draft PR, its very much a WIP for now.

[nmn]

The nextjs plugin will be overhauled soon since there are a lot of issues with our Webpack plugin and NextJS plugin. See the issue I created about this for information.

As such, it's not worth spending too much time adding types to the current NextJS plugin.

[lukewhitehouse]

Okay no worries, thanks for letting me know @nmn. I'll close this for now in that case.