Automatic `skipDefaultLibCheck` (and potentially `skipLibCheck`)

This issue is adapted from a work item discussed at #25658.

Automatic skipDefaultLibCheck

lib.d.ts is a pretty big file, and it's only going to grow. Realistically, most people don't ever declare symbols that conflict with the global scope, so we made the skipDefaultLibCheck (and also the skipLibCheck flag) for faster compilations.

We can suggest this flag to users, but the truth is that it's not discoverable. It's also often misused, so I want to stop recommending it to people. 😄

It'd be interesting to see if we can get the same results of skipDefaultLibCheck based on the code users have written. Any program that doesn't contribute a global augmentation, or a declaration in the global scope, doesn't really need to have lib.d.ts checked over again.

@mhegazy and I have discussed this, and it sounds like we have the necessary information after the type-checker undergoes symbol-merging. If no symbols ever get merged outside of lib files, we can make the assumption that lib files never need to get checked. But this requires knowing that all lib files have already had symbols merged up front before any other files the compiler is given.

Open Questions

• Is the idea to only type-check symbols which have been merged? Or is it to fall back to the worst-case behavior of checking everything when any symbol has been merged across files?
  • If it's the only checking merged entities, does declare class B extend A {} need to be re-checked if A has ever been subsequently merged? How would we keep track of that information?
  • If it's falling back to the worst-case behavior, does this feature actually help anyone outside of the "Hello world" case?

Cons

• It feels like there are certain interactions we may be overlooking - we will have to be careful about our assumptions in the compiler with this behavior in place.
• Users who edit lib.d.ts wouldn't see erroneous changes in a compiler (so we'd likely need a forceDefaultLibCheck).

Pros

• Running with skipDefaultLibCheck removes anywhere between 400-700ms on my machine from a "Hello world" file, so we could expect the same here under this behavior.
• Only our team ever needs to run forceDefaultLibCheck, reducing the cost for all other TypeScript users.

Read the type-checker symbol-merging behavior and how lib.d.ts files are loaded before user files. Trace the existing skipDefaultLibCheck and skipLibCheck behavior. Done means defining and validating a safe automatic-checking rule, including the open questions about merged symbols and a forceDefaultLibCheck escape hatch.