Support XNC file format for drill files

Ucamco just released the first version of the XNC format for drill files. It’s a subset of IPC-NC-349 (aka Excellon).

Data samples:

The first step towards improving the NC drill chaos is to develop a simple and clear
specification, without embellishments. Of course, no one wants a new format. There is already
an alternative format that solves all NC problems: Gerber. One wants a format that is
compatible in the sense that it can be read by all decent PCB drill input software. It must be a
subset of IPC-NC-349.

Would be nice if our drill file output would conform to the XNC format.