Fix require('ReactFeatureFlags') in eslint-plugin-react-hooks www build (#36243)

PR #35951 added FB_WWW_DEV builds for eslint-plugin-react-hooks to get
www-specific feature flag values. However, the FB_WWW build uses the
full ReactFeatureFlags.www.js fork, which contains:

  const dynamicFeatureFlags = require('ReactFeatureFlags');

This is a www Haste module that only exists in the www runtime. Rollup
can't tree-shake CJS require() calls (they're assumed side-effectful),
so the bare require('ReactFeatureFlags') survives in the build output
even though the eslint plugin only uses the static eprh_* exports.

When the built artifact is synced to www at
scripts/lint/eslint/rules/eslint-plugin-react-hooks/index.js, Node.js
fails with "Cannot find module 'ReactFeatureFlags'" because Haste
modules aren't available in the Node.js lint environment.

Create a dedicated fork (ReactFeatureFlags.eslint-plugin.www.js) that
exports only the static eprh_* flags with www values, without the
require('ReactFeatureFlags') dependency. Wire it up in forks.js for the
eslint-plugin-react-hooks entry point.

<!--
  Thanks for submitting a pull request!
We appreciate you spending the time to work on these changes. Please
provide enough information so that others can review your pull request.
The three fields below are mandatory.

Before submitting a pull request, please make sure the following is
done:

1. Fork [the repository](https://github.com/facebook/react) and create
your branch from `main`.
  2. Run `yarn` in the repository root.
3. If you've fixed a bug or added code that should be tested, add tests!
4. Ensure the test suite passes (`yarn test`). Tip: `yarn test --watch
TestName` is helpful in development.
5. Run `yarn test --prod` to test in the production environment. It
supports the same options as `yarn test`.
6. If you need a debugger, run `yarn test --debug --watch TestName`,
open `chrome://inspect`, and press "Inspect".
7. Format your code with
[prettier](https://github.com/prettier/prettier) (`yarn prettier`).
8. Make sure your code lints (`yarn lint`). Tip: `yarn linc` to only
check changed files.
  9. Run the [Flow](https://flowtype.org/) type checks (`yarn flow`).
  10. If you haven't already, complete the CLA.

Learn more about contributing:
https://reactjs.org/docs/how-to-contribute.html
-->

Co-authored-by: Eugene Choi <eugenechoi@meta.com>