Typechecker improvements

[iccir]

A lot of this work is already done, but I wanted to make an issue to document it.

• check-types should work with multiple Compiler objects when the Compiler#uses API is used. Historically, for performance reasons, we have recommended a single aggregate Compiler with all source files. This fails to catch certain types of dependency graph issues.
• We should use worker_threads to execute multiple copies of the TypeScript compiler in parallel.
• We should use the Language Services API and let TypeScript handle incremental compile logic.
• There should be an option to prevent Compiler#compile from waiting on type checker results. This would deliver the performance wins of the aggregate type checker while also catching the above-mentioned dependency graph issues.