[Fire] Add initial build infrastructure

[gaearon]

This adds:

• react-dom/unstable-fire entry point
• react-dom/src/fire folder which is where we'll put the implementation files
• A new host config in react-dom/src/fire/ReactFireHostConfig. Currently it just re-exports the old one, but you can verify it works by uncommenting the console.log in there.
• A new entry in bundles.js, forks.js, and a new inlining alias react-reconciler/inline.fire for our host config. This also means there's yarn flow fire now.
• A new entry point in react-dom/src/fire/ReactFire. It is a copy-paste of ReactDOM entry point with fixed relative paths. Its main purpose it just to show that the setup works — we'll fully replace the actual content later. Importantly, it references react-reconciler/inline.fire.
• Running yarn test-fire and yarn test-fire-prod now stubs react-dom with react-dom/unstable-fire. You can verify this by commenting out console.logs in the Fire host config and in the Fire entry point.
• There's a new fixture now. To get it working you'd need to run yarn build react/index,fire --type=UMD_DEV.

After this PR is merged we can rebase @trueadm's initial work on top.

Note: I decided to go with ReactFire rather than ReactDOMFire because we already know it's DOM and Dominic feels it's already getting too verbose for his taste. :P

[sizebot]

Details of bundled changes.

Comparing: d14ba87...f8e8430

react-dom

File	Filesize Diff	Gzip Diff	Prev Size	Current Size	Prev Gzip	Current Gzip	ENV
react-dom.profiling.min.js	0.0%	-0.0%	100.69 KB	100.69 KB	32.52 KB	32.52 KB	UMD_PROFILING
react-dom.profiling.min.js	0.0%	-0.0%	100.78 KB	100.78 KB	31.98 KB	31.97 KB	NODE_PROFILING
ReactDOM-prod.js	0.0%	-0.0%	308.18 KB	308.18 KB	56.77 KB	56.77 KB	FB_WWW_PROD
react-dom-unstable-fire.development.js	n/a	n/a	0 B	722.76 KB	0 B	166.98 KB	UMD_DEV
react-dom-unstable-fire.production.min.js	n/a	n/a	0 B	97.81 KB	0 B	31.85 KB	UMD_PROD
react-dom-unstable-fire.profiling.min.js	n/a	n/a	0 B	100.7 KB	0 B	32.53 KB	UMD_PROFILING
react-dom-unstable-fire.development.js	n/a	n/a	0 B	718.05 KB	0 B	165.61 KB	NODE_DEV
react-dom-unstable-fire.production.min.js	n/a	n/a	0 B	97.8 KB	0 B	31.39 KB	NODE_PROD
react-dom-unstable-fire.profiling.min.js	n/a	n/a	0 B	100.8 KB	0 B	31.98 KB	NODE_PROFILING
ReactFire-dev.js	n/a	n/a	0 B	738.37 KB	0 B	166.45 KB	FB_WWW_DEV
ReactFire-prod.js	n/a	n/a	0 B	296.78 KB	0 B	54.35 KB	FB_WWW_PROD
ReactFire-profiling.js	n/a	n/a	0 B	303.49 KB	0 B	55.8 KB	FB_WWW_PROFILING
react-dom-test-utils.production.min.js	0.0%	0.0%	9.97 KB	9.97 KB	3.71 KB	3.71 KB	UMD_PROD
react-dom-test-utils.development.js	0.0%	-0.0%	44.59 KB	44.59 KB	12.24 KB	12.24 KB	NODE_DEV
react-dom-test-utils.production.min.js	0.0%	0.0%	9.74 KB	9.74 KB	3.65 KB	3.65 KB	NODE_PROD
react-dom-unstable-native-dependencies.development.js	0.0%	0.0%	60.61 KB	60.61 KB	15.92 KB	15.92 KB	UMD_DEV
react-dom-unstable-native-dependencies.development.js	0.0%	-0.0%	60.29 KB	60.29 KB	15.79 KB	15.79 KB	NODE_DEV
react-dom-unstable-native-dependencies.production.min.js	0.0%	🔺+0.1%	10.75 KB	10.75 KB	3.7 KB	3.71 KB	NODE_PROD
ReactDOMServer-dev.js	0.0%	0.0%	121.18 KB	121.18 KB	31.79 KB	31.79 KB	FB_WWW_DEV
ReactDOMServer-prod.js	0.0%	0.0%	44.42 KB	44.42 KB	10.3 KB	10.3 KB	FB_WWW_PROD

Generated by 🚫 dangerJS

[trueadm]

Looks good – we also need a ReactFire specific ReactFlag if possible, like I've done in https://github.com/trueadm/react/tree/react-dom-fire so we can disable some legacy tests around thing we no longer support.

[philipp-spiess]

🔥

[gaearon]

we also need a ReactFire specific ReactFlag if possible

Let's do together with main integration PR.

[sebmarkbage]

So where did we land on the versioning here? We should be clear so that if someone has to do a patch release, they know what to do.

[gaearon]

Let's call next a minor. reactjs/react.dev#1447

[gaearon]

(Which probably means you should land yours too?)