Refactor RN walletlib build scripts

[Michaelsulistio]

This PR refactors the scripts in the package.json of RN walletlib. Previously, the library was using the rollup.config.ts script to create esm/cjs for browser/node/react-native environments.

For this library, we only need to support react native environment.

Changes:

• Added a dev dependency to https://github.com/callstack/react-native-builder-bob. This library provides convenient scripts to output a build lib for publishing.
• Refactor the package.json to use react-native-builder-bob rather than the rollup config.
• Add a README TODO list