Complete via TypeAtPos

[zth]

We've previously handled completion for let bindings where there's also a type annotation:

type user = {
  name: string,
  age?: int
}

let user: user = {name: "Test", <com>}

This gives completion because we look up that we're in a record called user.

However, this same snippet below should be valid if we've saved and compiled, even if there's no explicit type annotation:

type user = {
  name: string,
  age?: int
}

let user = {name: "Test", <com>}

let user will have the inferred type of user. But, this hasn't been picked up before.

This PR fixes that, by trying to complete for "the type at the position of the let binding" if there's no explicit type annotation. The cool part is that the compiler is quite liberal with assigning a type to the let binding even if it's not fully compiling. So, you could write a record expression without a type annotation, save and let the compiler infer what record it thinks you're trying to construct, and then get completions for that, even if the thing you're trying to construct isn't complete/doesn't type check yet.

Will make for a nicer DX when using inference.