assistant: Allow drag&dropping files/tabs into assistant panel (#17415)

This adds ability to the assistant panel's context editor to accept
files being dropped on it.

Multiple things can be dropped on the assistant panel:
- project panel entries (one or many)
- tabs (one)
- external files (one or many)


Release Notes:

- N/A


Demo:



https://github.com/user-attachments/assets/fddee751-cbdf-4e2c-ac80-35dfb857cc8a

Co-authored-by: Bennet <bennet@zed.dev>