npm start / npm build don't type check

[psvenk]

It turns out that Create React App's npm start and npm build don't check if the source type checks; they just run Babel and throw an error if the input is not syntactically valid TypeScript. We should run tsc somewhere (both for these scripts and for CI) instead of just relying on editor integrations and/or manually running tsc.

[cjquines]

i've basically never seen a project where the npm dev command runs typechecking or linting.

the build command i've sometimes seen runs the lint and typecheck