Feature: Output issues sorted and grouped by severity level

[bdsl]

When there are many issues, such as in a baseline or when adding Psalm to an existing project, it might be useful to see the issues in severity order and grouped by severity (i.e. which error level they would be displayed at).

This may help to get an understanding of the state of the project as well as to decide which error level to run Psalm at in future.

I might be able to find time to implement this if there's interest in it.

[bdsl]

Done in https://github.com/vimeo/psalm/releases/tag/5.1.0