Currently `readConfiguration` relies on the file system to perform disk
utilities needed to read determine a project configuration file and read
it. This poses a challenge for the language service, which would like to
use `readConfiguration` to watch and read configurations dependent on
extended tsconfigs (#39134). Challenges are at least twofold:
1. To test this, the langauge service would need to provide to the
compiler a mock file system.
2. The language service uses file system utilities primarily through
TypeScript's `Project` abstraction. In general this should correspond
to the underlying file system, but it may differ and it is better to
go through one channel when possible.
This patch alleviates the concern by directly providing to the compiler
a "ParseConfigurationHost" with read-only "file system"-like utilties.
For the language service, this host is derived from the project owned by
the language service.
For more discussion see
https://docs.google.com/document/d/1TrbT-m7bqyYZICmZYHjnJ7NG9Vzt5Rd967h43Qx8jw0/edit?usp=sharing
PR Close#39619
64c3135be7