adjacent let blocks should be joined

```elm
let
a = 1
in
let
b = 2
in
a + b
```

should merge the two blocks of definitions. Are there any cases where this changes the meaning of the code?

Reproduce the Elm example with elm-format and inspect how adjacent let/in expressions are currently formatted. Determine whether merging the definition blocks preserves meaning in relevant cases, then add coverage for the intended formatting behavior and its semantic edge cases.

Written by the indexing model from the issue text.