feat(kanata): add ultrawide mod

[Iorvethe]

This mod is in essence the wide mod applied to both hands, shifting them by one column each. This is super useful when using Arsenik on a regular staggered keyboard as the [LAlt] is quite uncomfortable to reach (as is the [RAlt] without the wide mod) and may in the long term cause RSI on the thumbs, which would be unfortunate for an ergonomic layout.

The advantages I see are:

• More comfortable [LAlt] which is extensively used
• Hands further apart, even closer to a split keyboard
• Potentially usable on keyboards with space-bars of >5 units

The possible drawbacks could be:

• Loss of all keys on the left side such as [Tab], [LShift], etc.
  • Possible only after the HRMs are enabled, in which case this left cluster is unused and could be removed
• Loss of home bumps, so harder to reposition without looking at the keyboard
  • May be solvable by either swapping the physical keys or by taping a DIY bump on the keys (what I did)
• In case of a narrow physical keyboard, the left hand could rest on an edge which would be uncomfortable

There are a few flaws with it, but the increased comfort is such that I think the mod has its place in the project. I have been daily-driving this mod for a few weeks and it is a significant improvement. For context, I have a 5 unit space-bar, which should be usable without the wide mods, but I still experienced pain in the base of the thumbs and the joints after a few days of usage. Migrating to the ultrawide mod solved the pain.

A few TODOs before merging (if the PR is accepted):

• Not sure about the name (or whether there is an agreed-upon name in the wider community for something similar)
• Add a note about the wide mods in the README and caution about the potential RSI by not using them