FR: jj op undo $n

[Zoybean]

Is your feature request related to a problem? Please describe.
Most often when I'm undoing something, I'll look at the op log and then figure out the number of operations I want to undo, then count that many -'s, e.g.: jj op restore @---.
I rarely have a use for more than 2 + or -, outside of jj op restore.

Describe the solution you'd like
I'd like to be able to say jj op undo 3 to undo the last 3 operations

Describe alternatives you've considered
jj op restore @-3 using the syntax in #4055 and #3337

[yuja]

See also #3700 for broader discussion.

[PhilipMetzger]

Yes, this should be folded into the new high level jj undo (#3700), as jj op undo is to low level for such a convenience detail