JS: Add ECMAScript 2024 v Flag Operators for Regex Parsing

[Napalys]

This pull request adds support for parsing ECMAScript 2024 v flag operators, including:

• Nested Classes: Enables using nested character classes in regexes.
  Example: /[[abc][cz]]/v
• Intersection (&&): Matches characters common to both sets.
  Example: /[[abc]&&[cz]]/v
• Subtraction (--): Removes characters from a set.
  Example: /[[abc]--[cz]]/v
  Mixing operations at the same level is not allowed:
  • Invalid: /[[abc]&&[cz]--[zz]]/v
  • Valid: /[[abc]&&[[cz]--[zz]]]/v
• Union: Combines multiple sets.
  Example: /[[abc][cz]]/v
• Quoted Strings (\q{}): Allows matching exact sequences.
  Example: /[\q{ab|cb|db}]/v

Commit by commit review encouraged.

Useful links:

• https://v8.dev/features/regexp-v-flag
• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/unicodeSets

With correct parsing, this no longer produces an false positive in Closes #18854.

[Copilot]

PR Overview

This pull request introduces support for ECMAScript 2024 regex constructs under the new "v" flag. Key changes include:

• New AST node classes for character class operations (Subtraction, QuotedString, Intersection, Union)
• Enhancements to RegExpParser to conditionally enable nested character classes, new operators, and quoted string parsing with a fallback mechanism when errors are encountered
• New test inputs covering quoted strings, unions, intersections, subtractions, and nested character classes

Reviewed Changes

File	Description
javascript/extractor/src/com/semmle/js/ast/regexp/CharacterClassSubtraction.java	New AST node for subtraction operator in character classes
javascript/extractor/src/com/semmle/js/ast/regexp/CharacterClassQuotedString.java	New AST node for handling quoted string escapes
javascript/extractor/src/com/semmle/js/ast/regexp/CharacterClassIntersection.java	New AST node for intersection operator in character classes
javascript/extractor/src/com/semmle/js/ast/regexp/CharacterClassUnion.java	New AST node for union operator in character classes
javascript/extractor/src/com/semmle/js/parser/RegExpParser.java	Extended parser functionality to support the new "v" flag and corresponding regex operations
javascript/extractor/src/com/semmle/js/extractor/ASTExtractor.java and RegExpExtractor.java	Updated extraction logic to accommodate new AST node types and conditional flag handling

Copilot reviewed 31 out of 31 changed files in this pull request and generated 2 comments.

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[asgerf]

Excellent work! I have a couple of comments to keep you busy during the week 😄