Fix StyleX CSS injection for Rspack unplugin

[tmkx]

What changed / motivation ?

Fixes the Rspack entry for @stylexjs/unplugin so StyleX CSS injection hooks are registered through the Rspack-specific hook.

unplugin invokes framework-specific hooks for Rspack via plugin.rspack(compiler). The StyleX Rspack entry reused the Webpack hook registration as plugin.webpack(compiler), which meant the JS transform could run while the CSS asset injection hook was skipped.

This updates the shared Webpack hook helper to accept the framework hook name, keeps webpack as the default, and makes the Rspack entry register the hook as rspack.

Pre-flight checklist

• I have read the contributing guidelines
  https://github.com/facebook/stylex/blob/main/.github/CONTRIBUTING.md
• Performed a self-review of my code

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