Add code action to wrap case clause body in braces

Add a code action that extracts the right hand side of a case clause into its own block.
Basically what "Add braces to arm expression" does in Rust.

Before:
```gleam
  case result {
    Ok(value) -> value + 1
                 ^^^^^^^^^
    Error(_) -> panic
  }
```
After:
```gleam
  case result {
    Ok(value) -> {
      value + 1
    }
    Error(_) -> panic
  }
```