[Mosaic GPU] Introduce a more flexible layout system

So far all of our layouts have been tailored to a limited set of use
cases we've tried so far, but they're still not general enough to
handle all of the register layouts needed for WGMMA or mixed precision
matmuls (incl. intermediate steps during conversions). Instead of adding
more special cases, I decided to adopt XLA tiled layouts and they do seem
to work quite well!

This change only lays the groundwork for the new layout system. Future
changes will build upon them to add new features and eventually replace
`WGMMA_LAYOUT` altogether.

PiperOrigin-RevId: 694105514