build: enable TypeScript

[anandaroop]

https://artsyproduct.atlassian.net/browse/FX-4123

This PR:

• lays down the plumbing to have TypeScript as part of the frontend build process for Rosalind
• updates the linting / testing / building setups accordingly
• does the bare minimum to convert the top-level App component from js → ts
• does not convert any other components

I've checked that all the following work:

• [x] local development with hot reloading, via Webpacker
• [x] yarn lint
• [x] yarn test
• [x] yarn type-check (new)
• [x] hokusai test
• [x] editor integration with VS Code

This is one of those stultifying infra PRs so I won't be offended if you just skim it 😅