Adjust pbox's margin based on length of prefix

[WardBrian]

Building on #96, this is a partial fix for #94.

The margin for pboxes is adjusted down based on the length of the prefix, meaning that text wraps in more expected ways when at the end of a long line.

It would, of course, be nice to also truncate the source line, which would also probably remove the need for the manual minimum width of 25.

Compare the test in this PR:

    error: cannot find value `missing` in this scope
        ┌─ foo.rs:2:140
      2 │    println!("What happens when I have an error at a really high column? Let's find out by trying to print something that doesn't exist {}", missing);
        │                                                                                                                                             ^^^^^^^ not found in this scope.
        │                                                                                                                                                      did you mean 'missingo'?

with rustc:

error[E0425]: cannot find value `missing` in this scope
 --> foo.rs:3:140
  |
3 | ...g that doesn't exist {}", missing);
  |                              ^^^^^^^ not found in this scope

[WardBrian]

There also might be an off-by-one somewhere, because I'm not sure why did you mean has one extra space to start out

Fixed by #98

[johnyob]

lgtm 🚀

Thank you for your contribution 🙏