feat: support path-aware exists and delete APIs

[mrousavy]

Summary

• add optional path arguments to existsMMKV(id, path?) and deleteMMKV(id, path?)
• pass root paths through to native MMKV::checkExist/MMKV::removeStorage
• expose public getBaseDirectory() so examples/tests do not import internal platform-context helpers
• keep custom-path Harness coverage native-only and remove the explicit Web negative-path test

Testing

• bun run specs
• bun run typecheck
• bun run lint
• bun run lint-ci
• bun run lint-cpp
• bun run test
• HARNESS_WEB_URL=http://localhost:3001 bun run test:harness --harnessRunner web
• ./gradlew :react-native-mmkv:assembleRelease --no-daemon

Upstream reference: Tencent MMKV exposes removeStorage/checkExist with an optional root path: https://github.com/Tencent/MMKV/blob/master/Core/MMKV.h#L749-L752