Experimental Event API: add event component mount phase callback

[trueadm]

This PR adds an onMount lifecycle event to event responders, in a similar way to the current onUnmount lifecycle. This is to be used in cases where things like <FocusScope> might want to set the initial focus of a DOM element in its scope upon mount. It works in the commit phase (a change from before) to ensure that the DOM tree has been committed at the same time.

Ref #15257

[sizebot]

ReactDOM: size: 0.0%, gzip: 0.0%

Details of bundled changes.

Comparing: 9c6ff13...6ca8cc5

react-dom

File	Filesize Diff	Gzip Diff	Prev Size	Current Size	Prev Gzip	Current Gzip	ENV
react-dom.development.js	+0.1%	0.0%	813.87 KB	814.51 KB	185.56 KB	185.62 KB	UMD_DEV
react-dom.production.min.js	0.0%	0.0%	103.61 KB	103.63 KB	33.64 KB	33.66 KB	UMD_PROD
react-dom.profiling.min.js	0.0%	0.0%	106.76 KB	106.78 KB	34.65 KB	34.65 KB	UMD_PROFILING
react-dom.development.js	+0.1%	0.0%	808.35 KB	808.99 KB	184.03 KB	184.08 KB	NODE_DEV
react-dom.production.min.js	0.0%	0.0%	103.6 KB	103.62 KB	33.1 KB	33.11 KB	NODE_PROD
react-dom.profiling.min.js	0.0%	0.0%	106.92 KB	106.95 KB	33.95 KB	33.95 KB	NODE_PROFILING
ReactDOM-dev.js	+0.1%	0.0%	832.65 KB	833.27 KB	185.38 KB	185.44 KB	FB_WWW_DEV
ReactDOM-prod.js	🔺+0.2%	🔺+0.1%	339.63 KB	340.2 KB	63.01 KB	63.07 KB	FB_WWW_PROD
ReactDOM-profiling.js	+0.2%	+0.1%	345.04 KB	345.61 KB	63.92 KB	63.99 KB	FB_WWW_PROFILING
react-dom-unstable-fire.development.js	+0.1%	0.0%	814.2 KB	814.83 KB	185.7 KB	185.75 KB	UMD_DEV
react-dom-unstable-fire.production.min.js	0.0%	0.0%	103.62 KB	103.64 KB	33.65 KB	33.67 KB	UMD_PROD
react-dom-unstable-fire.profiling.min.js	0.0%	0.0%	106.77 KB	106.79 KB	34.66 KB	34.66 KB	UMD_PROFILING
react-dom-unstable-fire.development.js	+0.1%	0.0%	808.68 KB	809.31 KB	184.16 KB	184.22 KB	NODE_DEV
react-dom-unstable-fire.production.min.js	0.0%	0.0%	103.61 KB	103.63 KB	33.11 KB	33.11 KB	NODE_PROD
react-dom-unstable-fire.profiling.min.js	0.0%	0.0%	106.94 KB	106.96 KB	33.96 KB	33.96 KB	NODE_PROFILING
ReactFire-dev.js	+0.1%	0.0%	831.83 KB	832.46 KB	185.38 KB	185.47 KB	FB_WWW_DEV
ReactFire-prod.js	🔺+0.2%	🔺+0.1%	327.63 KB	328.19 KB	60.6 KB	60.66 KB	FB_WWW_PROD
ReactFire-profiling.js	+0.2%	+0.1%	333 KB	333.57 KB	61.54 KB	61.6 KB	FB_WWW_PROFILING
react-dom-test-utils.development.js	0.0%	0.0%	54.19 KB	54.19 KB	14.98 KB	14.98 KB	UMD_DEV
react-dom-test-utils.production.min.js	0.0%	-0.0%	10.52 KB	10.52 KB	3.9 KB	3.89 KB	UMD_PROD
react-dom-test-utils.development.js	0.0%	0.0%	53.91 KB	53.91 KB	14.91 KB	14.91 KB	NODE_DEV
react-dom-test-utils.production.min.js	0.0%	-0.0%	10.31 KB	10.31 KB	3.82 KB	3.82 KB	NODE_PROD
react-dom-unstable-native-dependencies.development.js	0.0%	-0.0%	60.76 KB	60.76 KB	15.85 KB	15.85 KB	UMD_DEV
react-dom-unstable-native-dependencies.production.min.js	0.0%	-0.0%	10.69 KB	10.69 KB	3.67 KB	3.67 KB	UMD_PROD
react-dom-server.browser.development.js	0.0%	0.0%	136.73 KB	136.73 KB	35.92 KB	35.92 KB	UMD_DEV
react-dom-server.browser.production.min.js	0.0%	0.0%	19.18 KB	19.18 KB	7.21 KB	7.21 KB	UMD_PROD
react-dom-server.browser.development.js	0.0%	0.0%	132.86 KB	132.86 KB	34.99 KB	34.99 KB	NODE_DEV
react-dom-server.browser.production.min.js	0.0%	0.0%	19.1 KB	19.1 KB	7.2 KB	7.2 KB	NODE_PROD
ReactDOMServer-dev.js	0.0%	-0.0%	134.95 KB	134.95 KB	34.61 KB	34.61 KB	FB_WWW_DEV
ReactDOMServer-prod.js	0.0%	-0.0%	47.14 KB	47.14 KB	10.8 KB	10.8 KB	FB_WWW_PROD
react-dom-server.node.development.js	0.0%	0.0%	134.8 KB	134.8 KB	35.53 KB	35.54 KB	NODE_DEV
react-dom-server.node.production.min.js	0.0%	0.0%	19.96 KB	19.96 KB	7.51 KB	7.51 KB	NODE_PROD
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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