[style-guide] Better control of pattern match handlers and line breaks.

### Description

I would like to suggest having an option for pattern matching that could be set to one of four ways:

1. **Current behaviour**: Whatever this is exactly ;-)
2. **Prefer newlines**: Always have the clause handling on a new line and indented.
3. **Prefer same line**: If the pattern match handler is small enough, put it on the same line as the handler.
4. **Consistent** mode: Prefer same line, but *only if all matches can fit on the same line*. Otherwise, prefer newlines for all cases (i.e. fallback to option 2).

I'm not sure if this ideas are feasible or not - just thinking out aloud at a number of different styles we see in the wild (my personal preference is 4).