Fix crash on multibyte char in page ranges #241

[wrenger]

This fixes #241, which is a bug caused by incorrectly splitting the page ranges into groups.
The index calculation fails to account for a multibyte character at the end of a pages group. The subsequent split operation on the incorrect index leads to a crash, as described in the issue.

Examples:

@article{foo,
  ...
  pages     = {321--},
  ...
}
@article{bar,
  ...
  pages     = {321–},
  ...
}

Both page ranges are converted to a 321 followed by –, which is an utf8 character with multiple bytes

In addition to the fix, I've added a few tests for this and removed the string window in favor of the chars iterator from the std.

[wrenger]

Hi, seems pretty quiet around this repo. The focus is currently probably on the Typst compiler itself?

As this bug crashes the Typst compiler on valid BibTeX, it's quite critical for me, and I would be happy if it is fixed 😅