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>

crates/workspace/src/workspace.rs

@@ -2063,7 +2063,7 @@ impl Workspace {
         .detach_and_log_err(cx);
     }
 
-    fn project_path_for_path(
+    pub fn project_path_for_path(
         project: Model<Project>,
         abs_path: &Path,
         visible: bool,