Create forked rn_fusebox entry point

[huntie]

Summary

Adds a new rn_fusebox DevTools entry point, forked from rn_inspector . This entry point is designed to work with all features of the next-gen debugging stack (codename: React Fusebox ⚡).

Differences between entry points:

• rn_inspector.html — The current Experimental Debugger experience used in open source (RN + Expo), compatible with Hermes CDPHandler.
  • Changed in this PR — recent feature additions are removed, and are present in rn_fusebox.ts instead:
    • Host.RNPerfMetrics
    • ENABLE_REACT_DEVTOOLS_PANEL experiment
    • FuseboxClientMetadataModel
• rn_fusebox.html
  • Exact copy of rn_inspector prior to this PR.
  • We intend to more rapidly/aggressively tweak frontend features (without necessarily creating granular flags) using this new entry point.

Ideally, we will de-fork at some point in future and return to one rn_inspector.html entry point.

Note

This change should not be synced to React Native until we've configured the launch flow to conditionally select rn_fusebox.html.

Test plan

yarn build

New rn_fusebox.html entry point

✅ Renders, no unexpected console errors

rn_inspector.html entry point unchanged

✅ Renders, no unexpected console errors

Upstreaming plan

• This commit should be sent as a patch to the upstream devtools-frontend repo. I've reviewed the contribution guide.
• This commit is React Native-specific and cannot be upstreamed.

[EdmondChuiHW]

LGTM.

Small merge conflict with #33 but should be trivial.

Non-blocking: One thing it could be useful for us is *-meta.ts file/:meta build pattern. CDT convention seems to do setups there and code sharing. That way we can have the common code, e.g. experiment setup code, in the *-meta.ts file. Then the fusebox and rn_insepector entry points will be strictly distinct.

Note for the future: we can also add a response from FuseboxClient.setClientMetadata, so we can redirect/warn people in the UI if they somehow ended up in the Fusebox entrypoint without a supported target

[huntie]

@EdmondChuiHW

One thing it could be useful for us is *-meta.ts file/:meta build pattern. CDT convention seems to do setups there and code sharing.

Makes sense if we needed to maintain both entry points for the long term. In this case, the plan is to effectively freeze development on the old entry point and only touch rn_fusebox, until we delete the other one. Should only last one more major release cycle 🤞🏻. With this in mind, copying seemed pragmatic, especially given the less obvious forking of experiment defaults in the stacked changes.