Separate tsconfigs for test and build

[kimjoar]

As we want to use TypeScript in tests too, we can't exclude **/__tests__/** files from our production builds. The usual approach seems to be having the default tsconfig for a package include all files, then exclude tests in a separate tsconfig (and then specify --project to have it use that tsconfig when building the code).

[kimjoar]

This will likely get handled in the build system itself. I'll be writing up a larger issue on this stuff.

[epixa]

Closing in favor of #18780