FR: Add pad_center template function

[zx8]

Is your feature request related to a problem? Please describe.

Similar to the other issue I recently raised, I'm currently working through the surface-level features of jj to see if I can bring things in line with my git setup.

It does not seem possible to align text to the center of a column in the log output at the moment. Git has support for this via:

%><( <N> ), %><|( <M> )
similar to %<( <N> ), %<|( <M> ) respectively, but padding both sides (i.e. the text is centered)

ref: https://git-scm.com/docs/pretty-formats

Describe the solution you'd like

It would be nice to have a pad_center function, similar to pad_start and pad_end that middle-aligned text.

Describe alternatives you've considered

I'm not sure if there are any alternatives.

[martinvonz]

Or perhaps allow multiple templates and pad between them? pad_between(10, "-", "a", "b", "c") => a----b---c (or maybe a---b----c).

[martinvonz]

Then we would have:
pad_left(width, template, fill) = pad_between(width, fill, "", template)
pad_right(width, template, fill) = pad_between(width, fill, template, "")