follow tc39/ecma262#1869

[mysticatea]

This PR changes the regular expression validator in acorn in order to follow tc39/ecma262#1869.

Per consensus today, all of

/(?<\ud835\udc9c>.)/
/(?<\ud835\udc9c>.)/u
/(?<\u{1d49c}>.)/
/(?<\u{1d49c}>.)/u
/(?<𝒜>)/
/(?<𝒜>)/u

will be legal.

tc39/ecma262#1869 (comment)

Upstreams:

• Normative: Fully specify legal escape sequences in RegExp capture group names tc39/ecma262#1869
• tc39/test262 - not updated yet

[michaelficarra]

This was a spec bugfix. We can't say whether any capture group name was legal before 2020 because the spec was nonsensical. Thus, I don't think you should switch on the behaviour; just go with the es2020 clarification.