Embedder script + local serve script

[EdmondChuiHW]

Summary

• Add new embedderScript.js to entry point

Test plan

Test page load and the static files load in browser.

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.

[motiz88]

Recapping in-person discussion:

• The Python script is redundant - we should instead develop with a local checkout of @react-native/dev-middleware (fbsource) and have it serve the devtools-frontend checkout.
• Adding knowledge of the /debugger-frontend/ pathname to this repo is an antipattern. This repo should work just fine if served at whatever base URL.
• We should not change the Content-Security-Policy for the sake of a testing script.
• The README changes can mostly be reverted as a result of the above.
• We should also rename static/embedderScript.js to something like embedder-static/main.js.

[hoxyq]

LGTM. Consider also adding async or defer attributes.

2 questions:

1. Should it be strictly loaded before the entrypoint?
2. Can user see 404 error somewhere in the logs / UI?

[EdmondChuiHW]

• Should it be strictly loaded before the entrypoint?

Good point. There's potential for Expo to add global variables/custom behaviour here. But for now our use case can support async (as we check readyState and listen to DOMContentLoaded already). Amended

• Can user see 404 error somewhere in the logs / UI?

Yes it's visible in "dev tools in dev tools" console and network panels. Tho it has no impact on regular usage, I'll make a diff to maybe serve an empty file/status code by default to reduce the noise