[Pratt parser] Add EnablePrattParser option and dual-mode testing

[dmitriplotnikov]

Overview

• Extracted ANTLR parser implementation into parser/antlr_parser.go
• Added EnablePrattParser option in parser/options.go
• Added parser dispatcher in parser/parser.go to route between ANTLR and Pratt parsers
• Updated parser_test.go to run all test cases and benchmarks across both parsers
• Cleaned up redundant duplicate test cases from pratt_parser_test.go

Benchmark Results

Single-threaded (BenchmarkByCategory)

Benchmark Category	Implementation	Time (ns/op)	Memory (B/op)	Allocs/op	Speedup / Memory Reduction
Simple	ANTLR Pratt	325,234 ns/op 30,759 ns/op	110,000 B/op 13,549 B/op	1,646 allocs/op 404 allocs/op	~10.6x faster ~8.1x less memory (4.1x fewer allocs)
Complex	ANTLR Pratt	1,179,666 ns/op 124,332 ns/op	415,741 B/op 51,864 B/op	5,401 allocs/op 1,397 allocs/op	~9.5x faster ~8.0x less memory (3.9x fewer allocs)
Macros	ANTLR Pratt	422,952 ns/op 57,192 ns/op	138,618 B/op 23,930 B/op	2,049 allocs/op 641 allocs/op	~7.4x faster ~5.8x less memory (3.2x fewer allocs)
Errors	ANTLR Pratt	1,646,118 ns/op 99,719 ns/op	598,315 B/op 37,808 B/op	7,945 allocs/op 1,176 allocs/op	~16.5x faster ~15.8x less memory (6.8x fewer allocs)

Parallel Execution (BenchmarkParallelByCategory)

Benchmark Category	Implementation	Time (ns/op)	Memory (B/op)	Allocs/op	Speedup / Memory Reduction
Simple	ANTLR Pratt	199,177 ns/op 6,922 ns/op	110,128 B/op 13,494 B/op	1,649 allocs/op 404 allocs/op	~28.8x faster ~8.2x less memory
Complex	ANTLR Pratt	660,746 ns/op 27,752 ns/op	414,872 B/op 51,865 B/op	5,406 allocs/op 1,397 allocs/op	~23.8x faster ~8.0x less memory
Macros	ANTLR Pratt	219,749 ns/op 12,887 ns/op	138,100 B/op 23,732 B/op	2,053 allocs/op 641 allocs/op	~17.0x faster ~5.8x less memory
Errors	ANTLR Pratt	827,582 ns/op 21,358 ns/op	597,451 B/op 37,645 B/op	7,955 allocs/op 1,176 allocs/op	~38.7x faster ~15.9x less memory

[TristonianJones]

If the PrattParser and AntlrParser are internal details, we can keep the symbols unexported (lower-cased) and that will make it easier to control dependencies.