Collapse the output to 20 diagnostics maximum

[illright]

Ideally we should do it cleverly — if there are more diagnostics than the limit, we should make sure that we show as many unique issues as possible. Usually that means taking a diagnostic from each rule until we hit the limit and then printing it out, but still printing diagnostics from the same rule together.

Also we should mention in the footer of the output that the actual amount of diagnostics is larger, it's just that X of them were hidden