[Feature request] Provide "Relater" typechecker api

[artem1458]

Hey all!

It would be really convenient to have method isTypeAssignableTo from Relater (internal/checker/relater.go) provided in public api (js and possibly others?).

In current ts implementation (5.9) this method is internal, and some libraries using package @types/ts-expose-internals to use this method.
It can be used to implement a lot of different things at compile time, and I hope it's not a lot of work to expose and describe it.

Thanks!

[DanielRosenwasser]

We're investigating stuff around this. #2831 adds more of the foundation of type-space APIs. I anticipate we'll have relater APIs as well.