Provide API for faster incremental compilers

[alexeagle]

In #9017 (comment)
@mhegazy suggested the possibility that TS could include an API that makes it easy to build incremental compilers, similar to the --watch behavior of TSC.

Given a ts.Program object, and a string[] of files that were modified, re-read and parse only the changed files, getPreEmitDiagnostics only for the changed files, emit only the changed files, and give the updated Program.

Right now it's too much work for us to write our own tsc that has the --watch behavior.

cc @IgorMinar

[TheLarkInn]

+1 😏

[TheLarkInn]

Tagging @ahejlsberg per conversation we had today about this issue. This is the number one problem we have reported from those who are going from Babel to TypeScript and looking for fast incremental builds. Related issue #9017 for more context Anders.

[mhegazy]

Should be fixed by #20234