The Dot (.) without the s flag should not match U+2028 (LINE SEPARATOR)
and U+2029 (PARAGRAPH SEPARATOR) per the ECMAScript spec, not just \n
and \r. Use the existing is_line_terminator helper for consistency.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

- search() now saves/restores lastIndex so global/sticky regexps
always search from position 0 per the ES spec
- Remove 'v' (UnicodeSets) and 'd' (HasIndices) from lexer's accepted
regex flags since they're not implemented in the regex engine

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Add a from-scratch regex engine and wire it into the JS runtime:

- regex.rs: Pattern parser (character classes, quantifiers, anchors,
groups, alternation, backreferences, lookahead, escape sequences)
and backtracking matcher with proper continuation-based sequence
matching for greedy/lazy quantifiers.

- RegExp constructor and prototype methods (test, exec, toString)
with support for g/i/m/s/u/y flags, lastIndex tracking, and
named capture groups.

- Compiler emits RegExp(pattern, flags) calls for regex literals.

- String.prototype gains match, matchAll, search methods; replace,
replaceAll, split updated to handle RegExp arguments with capture
group substitution ($1, $&, etc.).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>