Implements basic focus-follows-mouse behavior.

Right now, it's only applied in the `workspace` crate for `Pane`s, so
anything that lives outside of that container (panels and such for the
most part) won't have this behavior applied. The core logic is
implemented as an extension trait, and should be trivial to apply to
other elements as it makes sense.



https://github.com/user-attachments/assets/d338fa30-7f9c-439f-8b50-1720e3f509b1



Closes #8167 

Release Notes:

- Added "Focus Follows Mouse" for editor and terminal panes

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Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>