Sort diagnostics by gravity (error -> warning -> info)

[poliorcetics]

Describe your feature request

When there are lots of warnings or infos in a project, the error diagnostics can be lost and hard to find in the diagnostic pickers.

They could be sorted by gravity to help go to the errors first.

[the-mikedavis]

We do this now for diagnostic icons in the gutter: #2827

I think it would be useful to have a way to sort the diagnostics picker by severity but I'm not sure we should sorting by severity by default. Sorting by location also seems like a reasonable default, or leaving it to whatever order the Language Server gives.

[poliorcetics]

To avoid changing anything in the sort there could be a marker that can be searched for in the view, lik [E] [W] and [I] maybe ? Files with collisions in those names will be very rare, and diagnostic text too (though here maybe it will collide a little more but that should be manageable)