Add alignment_mode argument to Span.char_span()

[itssimon]

Description

Adds the alignment_mode parameter present in Doc.char_span() to Span.char_span() as well.

Types of change

Enhancement

Checklist

• I confirm that I have the right to submit this contribution under the project's MIT license.
• I ran the tests, and all new and existing tests passed.
• My changes don't require a change to the documentation, or if they do, I've added all required information.

[adrianeboyd]

Thanks for the PR!

This also needs to be added to the API docs under website/docs/api/span.mdx. Add a tag in the Name column that this is new in 3.5.1 (<Tag variant="new">3.5.1</Tag>) and note that the docs are formatted using prettier, since you'll probably need to reformat the whole table.

[adrianeboyd]

Thanks again for the PR!