Add batched assignability checks into the Checker API #63873

Proposal

// existing
isTypeAssignableTo(source: Type, target: Type): boolean;

// batched - one result per pair, in order
areTypesAssignableTo(pairs: readonly (readonly [source: Type, target: Type])[]): boolean[];

// sugar for the two common shapes
areTypesAssignableTo(sources: readonly Type[], target: Type): boolean[];
areTypesAssignableTo(source: Type, targets: readonly Type[]): boolean[];

// short-circuiting quantifiers - one boolean, server stops early
isAnyTypeAssignableTo(sources: readonly Type[], target: Type): boolean;
areAllTypesAssignableTo(sources: readonly Type[], target: Type): boolean;

isTypeAssignableTo answers one pair per call, but in my use-case I usually have more than one pair to check.

What I actually do is match a type against a list of candidate types and take the ones that fit,
so a single run is candidates × requests checks. In my case - there could be thousands of such checks.

The API already batches like this in other places, getSymbolAtPosition, getSymbolAtLocation, getTypeAtLocation and getTypeOfSymbol all take arrays.

The short-circuiting quantifiers are also nice to have, but I understand that it complicates an api and I'm not sure it's really worth to have them since checker is already blazingly fast 😄

I've also prototyped the whole thing locally (Apple M4 Max, macOS 26.5.2, node v24.11.1, main@322fa1ecd):
1000 pairs cost 8.8 ms as 1000 sync calls versus 0.17 ms in one batched call, and 55 ms versus 0.24 ms on the async API.