Read tuple element labels

**Is your feature request related to a problem? Please describe.**

For documentation purposes I would like to get access to the [tuple element labels](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-0.html#labeled-tuple-elements), if any. Example:

```ts
const a: (() => [someLabel:1]) = [1]
```

**Describe the solution you'd like**

The function `t.getTupleElements()` could return more structured data. It currently just returns a `Type`. How about this:

```ts
interface TupleElementInfo {
label: null | string
type: Type
}
```

**Describe alternatives you've considered**

Given the current API maybe this would be consistent:

```ts
t.getTupleElementLabel()
```

But I find that weird since there's no guarantee that `t` is a tuple there...

Read CONTRIBUTING.md, then inspect the existing t.getTupleElements() API and how its Type result is consumed. Compare the proposed structured TupleElementInfo shape with the alternative label method using the labeled-tuple example. Done means tuple element labels, when present, are exposed consistently without breaking existing tuple type access.

Written by the indexing model from the issue text.