feat[devtools]: add package for fusebox integration (#28553)

## Summary

Stacked on https://github.com/facebook/react/pull/28552. Review only the
[last commit at the
top](https://github.com/facebook/react/pull/28553/commits/c69952f1bf6e23252d47e0f7eb98efbbb2cc2c55).

These changes add new package `react-devtools-fusebox`, which is the
entrypoint for the RDT Frontend, which will be used in Chrome DevTools
panel. The main differences from other frontend shells (extension,
standalone) are:
1. This package builds scripts in ESM format, this is required by Chrome
DevTools, see webpack config:

https://github.com/facebook/react/blob/c69952f1bf6e23252d47e0f7eb98efbbb2cc2c55/packages/react-devtools-fusebox/webpack.config.frontend.js#L50-L52
2. The build includes styles in a separate `.css` file, which is
required for Chrome DevTools: styles are loaded lazily once panel is
mounted.