Implement the beforeInput support detection

[oleq]

📝 Provide a description of the new feature

The event is supported in all major web browsers except Firefox (still an experimental feature behind a flag). This means for the time being both beforeInput and mutation typing will need to exist next to each other. And even after Firefox enables the feature, it will take some time for people to update their browsers (which is slow in corporate environments).

This is why isBeforeInputSupported = !env.isGecko does not make sense. We need a feature discovery similar to the one described on the React issue tracker instead.

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[Reinmar]

facebook/react#11211 and confirmation in https://stackoverflow.com/questions/49927038/how-to-feature-detect-input-events-level-1-or-2-support

[oleq]

Closed in #8021.