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 https://github.com/facebook/stylex/pull/60, closing https://github.com/facebook/stylex/issues/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.