Add Gerber and Excellon drill files

These are file types for transferring PCB fabrication data from the PCB layout program to the PCB manufacturers, and are based on the KiCad MIME data for the generic fabrication outputs. These are generic filetypes that are shared by many programs that do PCB design.

Only the Gerber file type is registered with the IANA (https://www.iana.org/assignments/media-types/application/vnd.gerber), but some applications already use the application/x-gerber identifier, so I added the IANA application/vnd.gerber as an alias to that.

The extension .gbr that is used by the registered Gerber file format is unfortunately also used by GIMP brushes, so the test has to ignore the glob matching and rely on the magic matching instead. Also, some programs use other extensions for Gerber files, so instead of globing on all those extensions, I have added in several magic lines based upon what I have seen other programs generating as the first line of the files (the exact contents of the first line aren't defined in the standard).