fix: enable SQLite RTree extension

[huytdps13400]

Summary

• enable SQLITE_ENABLE_RTREE for the bundled SQLite build on Android and iOS
• add a React Native harness regression test that creates, inserts into, and queries an RTree virtual table
• document that RTree is available by default

Closes #218.

Verification

• bun typecheck
• bun lint
• bun sqlite build
• bunx prettier --check README.md example/tests/unit/specs/operations/execute.spec.ts
• ruby -c packages/react-native-nitro-sqlite/RNNitroSQLite.podspec
• Android :react-native-nitro-sqlite:assembleDebug for all configured ABIs
• iOS RNNitroSQLite target build on iOS Simulator
• iOS harness: 33 unit tests, 1 TypeORM test, and 35 sqlite-vec tests passed
• bundled sqlite3.c native probe reproduced no such module: rtree without the flag and passed with the flag

Existing test-runner limitation

bun sqlite test --runInBand currently exits during Jest configuration because React Native 0.85 moved its Jest preset to @react-native/jest-preset. The native harness suites above exercise the changed behavior successfully.