`MultiBuffer::anchor_in_excerpt` currently just wraps the given text
anchor in a multibuffer anchor. This allows one to get a multibuffer
anchor that points outside its excerpt which is basically never what one
wants. This PR now does a bounds check and returns `None` if the given
text anchor is not within the bounds of the excerpt.
Release Notes:
- N/A *or* Added/Fixed/Improved ...
Co-authored-by: Kirill Bulatov <kirill@zed.dev>