Add filter_format field for Column in pickers

[peret]

Add the option to provide a formatting function for the filtering of picker columns that is separate from the formatting function for displaying it in the view.
Use this new option for the "path" column of the workspace diagnostics picker, so that we are filtering on the full (relative) file path, instead of the truncated value.

Fixes #13608

[the-mikedavis]

Before #9647 you could override the sort and filter text in pickers and we explicitly removed that. For a column with a path that is truncated really aggressively like the workspace diagnostics picker it makes some sense since you can guess what the truncated parts might be and try to filter on those. For other columns it doesn't really make sense: you can't always guess what's truncated. And as a consequence if you're filtering on text that is different than what is showed in the column then the highlighting of matching text can't work.

For #13608 we should change the way that paths are truncated so it's not so aggressive. The downside is that the picker is already very cramped so it takes up a lot of space.

[peret]

For #13608 we should change the way that paths are truncated so it's not so aggressive. The downside is that the picker is already very cramped so it takes up a lot of space.

Thanks for clarifying, I'll look into that 👍 As you point out, we don't have too much space for the path-column, so I'm wondering what would be a good way to truncate the path. Here's what I'm thinking right now: Have a character limit X for the path; last component (file name) is always displayed in full, the remaining characters are distributed equally to the remaining path components (at least one char per component).

Question is how to determine the character limit X. Constant value would be easiest, ofc. Is there a way to dynamically calculate a value that works? Is there maybe even an example how this is handled somewhere else in the code?