Create testing plan for the RegExp.escape proposal

[nicolo-ribaudo]

Proposal repo: https://github.com/tc39/proposal-regex-escaping

I plan to help @greg-nagy work on this later during the summer

[ptomato]

@nicolo-ribaudo @greg-nagy Your advice appreciated on how complete is https://github.com/tc39/test262/pull/4123

[bnoordhuis]

FWIW, I note that https://github.com/tc39/proposals doesn't mention a test262 flag but it's RegExp.escape

[ljharb]

Thanks, I've added it. Also, closing since #4123 landed.