(fix): beforeinput spellchecking bug on firefox #24882

[juank1809]

fixed react spellcheking on beforeinput bug

fixed spellcheking bug in firefox

fixed react spellcheking on beforeinput bug

fix spellchecking bug on firefox

Summary

This resolves https://github.com/facebook/react/issues/24358, in which the beforeInputEvent wasn't firing when triggering a spellchecking in firefox. The problem was in the getFallbackBeforeInputChars function: the spellchecking event is interpreted as an 'input' event in firefox, which the function wasn't handling.

How did you test this change?

I tested this change in the fixtures/packaging/babel-standalone/dev.html recreating the exact sandbox that creator of the issue built and the beforeInput fired as expected.