Add --side-by-side-fill-empty to fill up unmatched lines

[th1000s]

This changes

│ 10 │ deleted line   │    │                │
│    │                │ 11 │ added line     │

to the following. The fill char and style can be configured.

│ 10 │ deleted line   │    │ ╱╱╱╱╱╱╱╱╱╱╱╱╱╱ │
│    │ ╱╱╱╱╱╱╱╱╱╱╱╱╱╱ │ 11 │ added line     │

Thanks to @guanghechen for the idea (#2070).

---

  --side-by-side-fill-empty <CHAR_AND_STYLE>
      Fill empty side. Can be "true", or a fill character, or a character plus a style.

      On "true" the unmatched side will be filled with "/" in the style of
      --line-numbers-zero-style, if a single character is given this is used instead. If the
      character is followed by a style, that style is used.

---

Previews for dark and light mode:

I like it so much, I'm tempted to make it the default!

[guanghechen]

LGTM! @th1000s, thank you so much for your amazing work on this—it's truly appreciated! Any plans to merge this PR? I can't wait to start using it!

[dandavison]

Very nice. Is there an argument for just side-by-side-fill rather than side-by-side-fill-empty?