Add a document that describes differences with other regex engines

[BurntSushi]

On mobile, but here's a start:

• The impact of "unicode by default." E.g., character classes and word boundaries.
• Definition of line terminators.
• Duplicate capture group names.
• Legality of empty expressions.
• Alternation match precedence, e.g., leftmost first (Perl) vs leftmost longest (POSIX).
• Lack of certain features found in backtracking engines.
• Class set operations.
• Inconsequential escapes are not allowed.
• Differences between match semantics of ^/$.
• Less strictness on nonsensical repetitions. See #765.

[Absolucy]

Are duplicate capture group names still unsupported? ?(J) would be very useful for trying to merge several different subcases without straight up checking against multiple regexes

[BurntSushi]

Yes, they are still unsupported.