Allow selecting client-side or server-side trimming

@maintainers: Would be great to get some feedback, if you understand and like the idea in general and if it's okay to put it into the main module as a setting within the formatter or if it needs to be a submodule!

The base functionality and test coverage is there now, PHPUnit seems to fail due to an unrelated test from a different commit.

Okay, perhaps the pipeline failure isn't that unrelated, I will take a look.

Pipeline fixed!