refactor: move internal utils to packages/core

[dschlabach]

What changed? Why?
This is our first refactor for supporting RN. Internal utils feel like the best place to start since it's almost all pure logic

We'll make these changes incrementally, but eventually expect our src dir to look like:

src/
├── packages/
│   ├── core/          # Pure TS/JS logic
│   ├── react/         # React components
│   └── react-native/  # RN components

Notes to reviewers

How has it been tested?

[vercel]

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