diskdefs: Add format definitions for Xerox 860

[matmat]

Add Xerox 860 Word Processor Format Definitions

This PR adds format definitions for the Xerox 860 Word processor, enabling support for both single-sided and double-sided disk formats.

Tested with:

• Single-sided .IMD images from https://bitsavers.org/bits/Xerox/860/
  Note: Only the DM4*.IMD, 250Word*.imd, and SystemDisc6.3.imd files appear to be valid X860 images.
  The others are either not for Xerox 860 (VP*.imd) or incorrectly imaged (GenericPrograms6.2.imd).
• Double-sided .SCP images with private data (confirmed working but cannot be shared publicly)

Sector interleaving for Single-Sided disks:

• Cylinder 0: 1:1 interleave
• Cylinders 1-76: 1:2 interleave

This interleaving configuration matches factory-produced single-sided disks (based on examination of official Xerox disks with printed labels from bitsavers). While a few what seems to be user-created single-sided disks (with handwritten labels) use 1:2/1:4 interleaving, the implemented configuration likely represents the standard Xerox 860 single-sided format.

For double sided disks I have only ever seen 1:2/1:4 interleave configuration. Unfortunately none with official Xerox labels.

From marketing materials I have learned that there is also a double sided, double density floppy disk format availavle for the Xerox 860, but I have not access to any such disks.