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/project/src/terminals.rs

@@ -50,13 +50,7 @@ impl Project {
             .and_then(|entry_id| self.worktree_for_entry(entry_id, cx))
             .into_iter()
             .chain(self.worktrees(cx))
-            .find_map(|tree| {
-                let worktree = tree.read(cx);
-                worktree
-                    .root_entry()
-                    .filter(|entry| entry.is_dir())
-                    .map(|_| worktree.abs_path().clone())
-            });
+            .find_map(|tree| tree.read(cx).root_dir());
         worktree
     }