[Pratt parser] Port Pratt parser from cel/cpp

[dmitriplotnikov]

This PR ports the Pratt parser and hand-written lexer from github.com/cel-expr/cel-cpp/tree/master/parser/internal.

Highlights:

• PrattParser: Adds a recursive-descent top-down operator precedence (Pratt) parser implementation.
• Fast Lexer: Adds a dedicated lexer (parser/lexer.go) replacing ANTLR lexing with single-pass rune scanning.
• Performance: Pratt parser demonstrates significant performance improvements over ANTLR in BenchmarkParsers:
  • 8.4x – 14.8x faster execution time
  • 6.0x – 12.5x less memory consumed
  • 4.2x – 6.6x fewer memory allocations

This parser is still WIP and should not yet be used by clients of cel-go