Smarter errors around `Function.length`

I don't know how often other people have encountered this, but here goes: In the same vein as our uncalled function checks, most usages of Function.length are probably an error. Here's a toy example that just bit me:

declare function getResult(): string[];
for (let i = 0; i< getResult.length; i++) {
  getResult().slice(0, i).map(x => console.log(x));
}

Minimally, I think we could suggest that if a function returns something with a length property, and the function's length property is inspected instead, we could probably mark it as suspect.

Start by examining the existing uncalled function checks in the TypeScript compiler, then trace how function property access is analyzed. Define the intended diagnostic boundaries for inspecting Function.length when the function result has a length property, and add coverage showing when the usage is considered suspect.