Allow filtering by swift-tools-version

[AnthonyLatsis]

This sounds like a nice-to-have statistical qualifier in general. My personal interest as a compiler engineer lied in finding packages that use the latest features, i.e., swift-tools-version: 6.2.

Previously discussed as part of #1403 a few years ago. I figured it would still be helpful to file a separate request.

[finestructure]

I wonder how useful this actually would be in practice. We see the tools version mostly change as the default changes - i.e. with a new Swift version, new packages will have that version's tools-version by default. People then sometimes adjust this to their needs but any queries across the whole ecosystem will be entirely dwarfed by the default at each Swift version 🤔

Can you think of a query you'd be interested in? I could run it and we can see if the results are useful at all as a way of testing before implementing.