Clarify that \R matches UTS#18 definition of line boundaries

[mathiasbynens]

This doesn’t affect what’s being proposed, but it does make it explicit that this matches UTS#18. This seems nice, especially since [the RegExp v flag proposal has a goal of matching UTS#18 more and more](see tc39/proposal-regexp-v-flag#42 and in particular tc39/proposal-regexp-v-flag#43), and this kind of directional alignment between proposals seems desirable.

[markusicu]

+1 tnx

[RunDevelopment]

I have a question regarding the equivalent regex.

From what I understand, the goal of UTS#18 Line Boundaries section is to say that CRLF should be treated as if it was a single character. However, I believe that neither the current equivalent regex in this proposal ((?>\r\n?|[\x0A-\x0C\x85\u{2028}\u{2029}])) nor the one in UTF#18 ((?:\u{D A}|(?!\u{D A})[\u{A}-\u{D}\u{85}\u{2028}\u{2029}])) fulfill that goal.

The problem I see is that they still match in between CR and LF (see tc39/proposal-regexp-v-flag#42). Consider /^\r\R$/u.test("\r\n"). According to the current proposal and UTS#18, this will return true. I think this is inconsistent with the behavior of line boundary assertions (^$) in UTS#18 where the position in between CR and LF is explicitly accounted for.

Shouldn't the equivalent regexes for this proposal and UTS#18 be (?>\r\n?|(?<!\r)\n|[\x0B-\x0C\x85\u{2028}\u{2029}]) and (?:\u{D A}|(?!\u{D A})[\u{A}-\u{D}\u{85}\u{2028}\u{2029}](?<!\u{D A})) respectively?

[rbuckton]

UTS#18 affects how ^, $, and . match as well. I would argue that \R should be consistent with how ^ and $ (at least) match depending on mode. If ^ or $ can match in between a CRLF with the u flag, but not with the v flag, then perhaps \R should match the behavior in each mode (i.e., allow matching in between CRLF in u mode but not in v mode). That said, \R is a new construct under either mode, so we could diverge from ^ and $ if that's reasonable.

[rbuckton]

@RunDevelopment I've referenced your comment in #2 so that it can be discussed further outside of this PR.