feat: add RegexpCompiler option for custom regexp engines

[satisataka]

Allow users to provide a custom regexp compiler via ResolveOptions.RegexpCompiler for PCRE-compatible features like lookahead that Go's regexp package does not support.

The Regexp interface requires only MatchString(string) bool, matching both regexp.Regexp and regexp2/compat.Regexp signatures.

Fixes #72

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[karolpiotrowicz]

@wolo-lab — routing this to you for a direction call rather than a code review.

This adds ResolveOptions.RegexpCompiler and a small Regexp interface so a caller can substitute a different regular-expression engine, for lookahead and other PCRE features Go's regexp does not support. Issue #72 asks for it.

The scope question is conformance rather than code quality. JSON Schema 2020-12 §6.4 says regular expressions SHOULD be valid per ECMA-262 and asks schema authors to stay inside a restricted token set, specifically so schemas stay portable between implementations. A pluggable engine lets a schema depend on syntax no other validator will accept, so it is a decision about what this library guarantees, not about whether the option is well implemented.

adk-go depends on this module, which is why it is coming to you. Open 134 days. Happy to run a full review once there is a direction.