Closes rvben#644.

MD030 enforces a fixed number of spaces after each list marker. That
keeps text aligned while every marker is the same width, but the text
column jumps as soon as an ordered list crosses a digit boundary (`9.`
-> `10.`):

```markdown
1. one
9. nine
10. ten
```

Add an opt-in `ol-align-column` setting that lines ordered list text up
at a fixed column instead:

```toml
[MD030]
ol-align-column = 4   # 0 = off (default)
```

```markdown
1.  one
9.  nine
10. ten
```

## Behavior details

- The column is measured from the start of the marker. A narrower marker
  is padded up to it (overriding `ol-single`/`ol-multi` for ordered
  items); a marker too wide for it overflows with a single space rather
  than pushing the shorter entries over, so the common one- and
  two-digit items stay aligned, but we don't create wider spacing that
  might not even be correct.
- Padding is capped at 4 spaces, since 5+ spaces after a marker start an
  indented code block in CommonMark.
- The column is relative to each marker's own start, so nested lists
  align independently of their parent. Off by default (`0`), so existing
  behavior is unchanged.

Docs (docs/md030.md) and unit + integration tests are included. The
tests use `indoc!` so the expected indentation is visible in-source.

Assisted-by: Claude