feat: make pcre2 an optional cargo feature with docs

[biswapanda]

Add [features] section to Cargo.toml with pcre2 as an optinal and default feature.

• Gate all pcre2-related code in split.rs behind #[cfg(feature = pcre2)] so consumers can opt out with default-features = false.

• Add feature-gated tests for both pcre2-on and fallback paths.

• Document the feature flag in README.md and add CONTRIBUTING.md with build/test instructions.

Related Dynamo CI failure - https://github.com/ai-dynamo/dynamo/actions/runs/23101799392/job/67103786703

This will allow disabling strict pcre2 (libpcre2-dev) dependency -

=================================== FAILURES ===================================
_______________________ test_no_bundled_shared_libraries _______________________
tests/basic/test_wheel_contents.py:34: in test_no_bundled_shared_libraries
    assert (
E   AssertionError: Unexpected shared libraries bundled in ai-dynamo-runtime:
E       ai_dynamo_runtime.libs/libpcre2-8-516f4c9d.so.0
E   assert not ['ai_dynamo_runtime.libs/libpcre2-8-516f4c9d.so.0']
        bundled_libs = ['ai_dynamo_runtime.libs/libpcre2-8-516f4c9d.so.0']
        installed_files = [PackagePath('ai_dynamo_runtime-1.0.0.dist-info/INSTALLER'), PackagePath('ai_dynamo_runtime-1.0.0.dist-info/METADATA')...Path('ai_dynamo_runtime-1.0.0.dist-info/WHEEL'), PackagePath('ai_dynamo_runtime-1.0.0.dist-info/direct_url.json'), ...]