Add internal logic for listening to event responders

[trueadm]

Note: this is for an experimental event API that we're testing out internally at Facebook.

This PR adds logic to listen to events defined in event responders, following on from previous PRs:

• Add infrastructure for passive/non-passive event support for future API exploration #15036
• Adds more scaffolding for experimental event API #15112
• Adds react-events package for internal testing #15150

This PR is quite difficult to test in isolation without creating unit tests that depend on internals (which I know we're moving away from as a team). In the next PR, I'll add further logic that connects the event firing to appropriate events that make for good public API tests. There isn't much to this PR to review other than a single function where most of the work happens, so hopefully it should be transparent as to what happens.l

[threepointone]

just nits, but looks good to me. and I understood it this time!

[sizebot]

Details of bundled changes.

Comparing: c05b4b8...f9fa91b

react-dom

File	Filesize Diff	Gzip Diff	Prev Size	Current Size	Prev Gzip	Current Gzip	ENV
react-dom.development.js	+0.5%	+0.5%	796.63 KB	800.26 KB	180.8 KB	181.69 KB	UMD_DEV
react-dom.profiling.min.js	0.0%	-0.0%	108.08 KB	108.08 KB	34.62 KB	34.62 KB	UMD_PROFILING
react-dom.development.js	+0.5%	+0.5%	791.02 KB	794.65 KB	179.19 KB	180.07 KB	NODE_DEV
react-dom.profiling.min.js	0.0%	-0.0%	108.17 KB	108.17 KB	34.07 KB	34.07 KB	NODE_PROFILING
ReactDOM-dev.js	+0.5%	+0.6%	814.97 KB	818.92 KB	180.57 KB	181.58 KB	FB_WWW_DEV
ReactDOM-prod.js	🔺+0.7%	🔺+0.6%	322.21 KB	324.47 KB	59.24 KB	59.6 KB	FB_WWW_PROD
ReactDOM-profiling.js	+2.7%	+2.2%	327.18 KB	336.02 KB	60.48 KB	61.79 KB	FB_WWW_PROFILING
react-dom-unstable-fire.development.js	+0.5%	+0.5%	796.98 KB	800.6 KB	180.93 KB	181.83 KB	UMD_DEV
react-dom-unstable-fire.production.min.js	0.0%	-0.0%	105.11 KB	105.11 KB	33.95 KB	33.95 KB	UMD_PROD
react-dom-unstable-fire.profiling.min.js	0.0%	-0.0%	108.09 KB	108.09 KB	34.63 KB	34.63 KB	UMD_PROFILING
react-dom-unstable-fire.development.js	+0.5%	+0.5%	791.37 KB	794.99 KB	179.33 KB	180.21 KB	NODE_DEV
react-dom-unstable-fire.production.min.js	0.0%	-0.0%	105.08 KB	105.08 KB	33.47 KB	33.47 KB	NODE_PROD
react-dom-unstable-fire.profiling.min.js	0.0%	-0.0%	108.18 KB	108.18 KB	34.08 KB	34.08 KB	NODE_PROFILING
ReactFire-dev.js	+0.5%	+0.5%	814.18 KB	818.13 KB	180.45 KB	181.44 KB	FB_WWW_DEV
ReactFire-prod.js	🔺+0.7%	🔺+0.7%	310.79 KB	313.06 KB	56.87 KB	57.28 KB	FB_WWW_PROD
ReactFire-profiling.js	+2.8%	+2.0%	315.82 KB	324.56 KB	58.14 KB	59.32 KB	FB_WWW_PROFILING
react-dom-test-utils.development.js	0.0%	-0.0%	47.21 KB	47.21 KB	12.91 KB	12.91 KB	NODE_DEV
react-dom-unstable-native-dependencies.development.js	0.0%	-0.0%	60.53 KB	60.53 KB	15.8 KB	15.8 KB	UMD_DEV
react-dom-unstable-native-dependencies.production.min.js	0.0%	-0.0%	10.69 KB	10.69 KB	3.67 KB	3.66 KB	UMD_PROD
react-dom-unstable-native-dependencies.development.js	0.0%	-0.0%	60.2 KB	60.2 KB	15.67 KB	15.67 KB	NODE_DEV
react-dom-unstable-native-dependencies.production.min.js	0.0%	-0.1%	10.42 KB	10.42 KB	3.57 KB	3.56 KB	NODE_PROD
react-dom-server.browser.development.js	0.0%	-0.0%	133.58 KB	133.58 KB	35.32 KB	35.32 KB	UMD_DEV
react-dom-server.browser.development.js	0.0%	-0.0%	129.71 KB	129.71 KB	34.39 KB	34.39 KB	NODE_DEV
ReactDOMServer-dev.js	0.0%	-0.0%	131.94 KB	131.94 KB	34.08 KB	34.08 KB	FB_WWW_DEV
react-dom-server.node.development.js	0.0%	-0.0%	131.65 KB	131.65 KB	34.93 KB	34.93 KB	NODE_DEV
react-dom-unstable-fizz.browser.production.min.js	0.0%	-0.3%	1.21 KB	1.21 KB	706 B	704 B	UMD_PROD
react-dom-unstable-fizz.browser.production.min.js	0.0%	-0.2%	1.05 KB	1.05 KB	637 B	636 B	NODE_PROD
react-dom-unstable-fizz.node.development.js	0.0%	-0.1%	3.74 KB	3.74 KB	1.43 KB	1.43 KB	NODE_DEV
react-dom-unstable-fizz.node.production.min.js	0.0%	-0.1%	1.1 KB	1.1 KB	667 B	666 B	NODE_PROD

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