Type-safe query source generator support

I wanted to do compile-time caching, but at the moment it's quite difficult to achieve.

Compilation itself is possible with a source generator, but it is difficult to resolve all of the project's dependencies and compile it.

It could be done by caching only the first time it is run, or some other similar technique, but if it's only to that extent, I don't think it's worth making a library.

I'd like to keep the idea as it is, but it's quite low priority.