[misc] vim: Hygenic text width highlights

[misc] vim: Hygenic text width highlights

This lets Factor's overly long line highlighting avoid bleeding over
into documentation source buffers, or buffers of other non-Factor file
types entirely. Also, by taking `:2match` instead of `:match`,
clobbering of most user matches (or vice versa) can be avoided.

Unfortunately, the highlighting effects all windows in a multi-buffer
split setup, but since we can't reasonably make this a `:syntax match`
group, it'll have to do. (And this behavior isn't new.)