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/tasks_ui/src/modal.rs

@@ -68,7 +68,7 @@ impl TasksModalDelegate {
         };
         Some((
             source_kind,
-            new_oneshot.resolve_task(&id_base, &self.task_context)?,
+            new_oneshot.resolve_task(&id_base, Default::default(), &self.task_context)?,
         ))
     }
 
@@ -684,6 +684,7 @@ mod tests {
 
         cx.dispatch_action(Spawn {
             task_name: Some("example task".to_string()),
+            target: None,
         });
         let tasks_picker = workspace.update(cx, |workspace, cx| {
             workspace