Add more non-React events to the priority list

[gaearon]

In this PR, I'm adding more events to the priority switch. I went through MDN Events and added a few Discrete events and a few Continuous ones.

This includes events that we polyfill (like mouseenter and mouseleave), the ones where we use different names (like focus and blur), and some that we never had in the list but probably should now (like popstate — #17279).

Added a test for mouseover (already in the list) and mouseenter (new in the list) inspired by a similar test using React handlers from ChangeEventPlugin-test.

[sizebot]

Comparing: d919e2c...9c07beb

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.min.js	+0.14%	122.31 kB	122.48 kB	+0.08%	39.42 kB	39.45 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	+0.13%	128.89 kB	129.06 kB	+0.09%	41.46 kB	41.50 kB
facebook-www/ReactDOM-prod.classic.js	+0.05%	406.62 kB	406.84 kB	+0.07%	75.37 kB	75.42 kB
facebook-www/ReactDOM-prod.modern.js	+0.06%	394.97 kB	395.19 kB	+0.06%	73.49 kB	73.54 kB
facebook-www/ReactDOMForked-prod.classic.js	+0.05%	406.63 kB	406.85 kB	+0.07%	75.37 kB	75.42 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

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