Add offset method to ruff_python_trivia::Cursor

[AlexWaygood]

Summary

Two Parser structs in the ty_test crate have offset() and skip_whitespace methods that are identical in their implementations. These methods feel like they could live just as well on the ruff_python_trivia crate's Cursor struct, which would reduce code duplication. (Edit: following code review, this PR now only adds the offset method to the Cursor struct.)

The short-term motivation for doing this is that I found myself needing them yet again for another parsing task in #18057 -- but it seems like a reasonable thing to do anyway, so I'm splitting it out into a standalone PR.

Test Plan

cargo test -p ty_test

[github-actions]

mypy_primer results

No ecosystem changes detected ✅

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

Formatter (stable)

✅ ecosystem check detected no format changes.

Formatter (preview)

✅ ecosystem check detected no format changes.

[MichaReiser]

ty