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/workspace/src/workspace.rs

@@ -686,7 +686,9 @@ pub enum Event {
     },
     ContactRequestedJoin(u64),
     WorkspaceCreated(WeakView<Workspace>),
-    SpawnTask(Box<SpawnInTerminal>),
+    SpawnTask {
+        action: Box<SpawnInTerminal>,
+    },
     OpenBundledFile {
         text: Cow<'static, str>,
         title: &'static str,