Sort "let-imports" with "dhall lint"

I like to keep my "external let expressions", i.e. "imports", at the top of each file where they're being used. It would be nice if `dhall lint` could fix that automatically.

Example:

```
let x = "foo"
let List/concat = https://prelude.dhall-lang.org/List/concat.dhall
```

->

```
let List/concat = https://prelude.dhall-lang.org/List/concat.dhall

let x = "foo"
```