Better auto indentation

```haskell
a = do
let b = do
-- cursor should go here
-- not here
```
In the example, `let` can be changed to `where`, `do` can be changed to `case x of`. There may be more such combinations to consider.

No file or test is named. Start by locating the extension's existing Haskell auto-indentation rules and reproduce the nested do/let example; compare the expected cursor position with the current behavior and check the mentioned where and case x of combinations. Done means the shown indentation and the additional supported combinations behave correctly.

Written by the indexing model from the issue text.