Add more tests for regex BNF

[macchiati]

Add more code to test the UTS #18 EBNF

[macchiati]

Adds some files that do parsing at a higher level (not as optimized as UnicodeSet) to make it easier to make changes. Use it to test proposed UTS #18 EBNF updates.

[markusicu]

Is this still relevant? Should I assign this to myself for review?

I think I looked at this some time ago and had some concerns about duplicating code from ICU class UnicodeSet... or was that a different discussion?

[markusicu]

@macchiati reminder of an old project...

[markusicu]

Hi @macchiati it looks like there are two sets of changes here:

• Some changes in emoji-proposal-handling code, adding some extra checks?
• An additional UnicodeSet parser related to the TC39 ECMAScript regex proposal?

Are you intending to take one or both of these into the main branch eventually?

@nedley FYI something to grab from the emoji code changes here?

[macchiati]

This was during investigations of regex sets of strings. I'll close it.