Add a full JavaScript lexer to the we-js crate supporting all ES2024 token types: keywords, identifiers (including Unicode), numeric literals (decimal, hex, octal, binary with separators), string literals with full escape sequences, template literals with ${} substitution tracking, regular expression literals with context-based disambiguation, and all punctuators/operators. Includes source position tracking (line/column) and newline tracking for automatic semicolon insertion. 48 unit tests covering all token types, edge cases, and error handling. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>