This is a small PR to get some eyeballs and hopefully some feedback on these changes. As #14566 rightfully points out, it's unintuitive that exact matches are shown last after all the partial matches.

I just did the most straightforward thing, which is to check if the pattern exactly matches the text, and give that a higher priority sort over anything else. It also seems natural to me to display smaller matches first, but I don't think this is that important.

Closes #14566

Note: I wanted to add tests for this but AFAICT there are no tests currently which exercise the sorting of completion items. I was going to add my own, but since the Completion struct takes an Editor, and I couldn't find any tests which actually initialized a whole Editor instance, I wasn't sure if this was the right approach.