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/task_store.rs

@@ -331,7 +331,7 @@ fn local_task_context_for_location(
     let worktree_id = location.buffer.read(cx).file().map(|f| f.worktree_id(cx));
     let worktree_abs_path = worktree_id
         .and_then(|worktree_id| worktree_store.read(cx).worktree_for_id(worktree_id, cx))
-        .map(|worktree| worktree.read(cx).abs_path());
+        .and_then(|worktree| worktree.read(cx).root_dir());
 
     cx.spawn(|mut cx| async move {
         let worktree_abs_path = worktree_abs_path.clone();