feat: add kernel switching for Microsoft Surface devices

[8bitbuddhist]

Description of changes

This PR adds the ability to choose between the most recent long term support kernel and the most recent stable kernel for Microsoft Surface devices (6.12 and 6.13 as of this writing). This originated from this PR and the need for some users to stay on older kernel versions while waiting for modules to catch up. This also makes kernel updates slightly easier, since the versions and hashes have been moved into a single file (microsoft/surface/common/default.nix).

Things done

• Tested the changes in your own NixOS Configuration (currently using it with the stable kernel)
• Tested the changes end-to-end by using your fork of nixos-hardware and
  importing it via <nixos-hardware> or Flake input

[8bitbuddhist]

I forgot to separate the source kernel version and the linux-surface version. Testing out the fix now.