Create monorepo combining mobile-sdk-core, mobile-ui-react, and mobile-samples

[tcobbs-bentley]

The mobile-sdk-core, mobile-ui-react, and mobile-samples repositories should be combined into a single pnpm-based monorepo to make development easier. This would remove the npx relative-deps requirement that currently exists when doing modifications that cross the repository boundaries.

Note: there are also mobile-sdk-ios and mobile-sdk-android repositories that contain iOS and Android native code. I don't think those could or should be moved into the monorepo.