ruff_annotate_snippets: support overriding the "cut indicator"
We do this because `...` is valid Python, which makes it pretty likely
that some line trimming will lead to ambiguous output. So we add support
for overriding the cut indicator. This also requires changing some of
the alignment math, which was previously tightly coupled to `...`.
For Ruff, we go with `…` (`U+2026 HORIZONTAL ELLIPSIS`) for our cut
indicator.
For more details, see the patch sent to upstream:
https://github.com/rust-lang/annotate-snippets-rs/pull/172
