feat: Add predicate support for node text filtering

[zharinov]

Summary

• Add 7 predicate operators for node text filtering: ==, !=, ^=, $=, *=, =~, !~
• Compile regex predicates to sparse DFAs via regex-automata
• Validate regex patterns for unsupported features (backreferences, lookaround, named captures)

Why

Predicates enable filtering nodes by their text content, a key feature for precise code queries. String predicates handle common cases efficiently, while regex predicates provide full Unicode pattern matching.

Syntax

(identifier == "foo")           ; equals
(identifier ^= "get")           ; starts with
(identifier =~ /^test_/)        ; regex match