Investigate Adopting `--isolatedDeclarations` in TypeScript's Codebase

[DanielRosenwasser]

We would like to enable the isolatedDeclarations flag on the TypeScript codebase. One reason for this is simple dog-fooding (i.e. making sure we understand what users are experiencing). Another is to enable faster builds, whether that be through parallelized declaration emit, or through handing declaration emit off to a faster build tool.

First thing's first though - enable the flag, see what this feels like, and understand the pain points.

[escaton]

I came across this by accident, but have a suggestion to at least try oxc-transform for d.ts generation. I found it to be the fastest and closest to what tsc emits on my project (20k TS files).