Fix duplicate named capturing groups and clean up regex test262 exclusions

[lahma]

Summary

• Fix RegExpPrototype groups object construction for duplicate named capturing groups: pre-initialize all unique group names with undefined in source order, then only update when a capture actually matches. This prevents non-participating groups from overwriting successful captures with undefined. Applies to exec, match, matchAll, replace, and indices.
• Remove regexp-lookbehind feature exclusion — most lookbehind tests already pass, only 2 individual files excluded for .NET-specific capture semantics in lookbehind backreferences.
• Clean up regex exclusions: use glob pattern for CharacterClassEscapes, remove duplicate entries, add proper categorization comments for each exclusion group.

Net result: +38 passing test262 tests (92,410 → 92,448), 0 failures, no Acornima changes needed.

Duplicate named groups fix

The core issue: when building the groups object for regex match results, iterating groups in order and calling CreateDataPropertyOrThrow for each caused non-participating groups (from unmatched alternatives) to overwrite successful captures with undefined.

Example: /(?<x>a)|(?<x>b)/.exec("b") — group 1 ((?<x>a)) doesn't match, group 2 ((?<x>b)) matches "b". Previously, processing group 1 first would set groups.x = undefined, then group 2 would set groups.x = "b" — but only if using CreateDataPropertyOrThrow which creates a new property each time. The fix uses a two-phase approach: first pre-initialize all unique names, then only update on successful captures.

6 tests with quantified groups ({2}) remain excluded due to .NET's System.Text.RegularExpressions retaining captures from previous iterations (JavaScript resets them).

Test plan

• Full test262 suite: 0 failures, 92,448 passed
• Jint.Tests: 0 failures
• No .csproj, .slnx, or Directory.Packages.props changes — uses released Acornima 1.2.0

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