Add the ability for tasks to target the center pane (#22004)

Closes #20060
Closes #20720
Closes #19873
Closes #9445

Release Notes:

- Fixed a bug where tasks would be spawned with their working directory
set to a file in some cases
- Added the ability to spawn tasks in the center pane, when spawning
from a keybinding:

```json5
[
  {
    // Assuming you have a task labeled "echo hello"
    "ctrl--": [
      "task::Spawn",
      { "task_name": "echo hello", "target": "center" }
    ]
  }
]
```

crates/worktree/src/worktree.rs

@@ -2533,6 +2533,12 @@ impl Snapshot {
         self.entry_for_path("")
     }
 
+    pub fn root_dir(&self) -> Option<Arc<Path>> {
+        self.root_entry()
+            .filter(|entry| entry.is_dir())
+            .map(|_| self.abs_path().clone())
+    }
+
     pub fn root_name(&self) -> &str {
         &self.root_name
     }