Move task centering code closer to user input (#22082)

Follow-up of https://github.com/zed-industries/zed/pull/22004 

* Reuse center terminals for tasks, when requested
* Extend task templates with `RevealTarget`, moving it from
`TaskSpawnTarget` into the core library
* Use `reveal_target` instead of `target` to avoid misinterpretations in
the task template context
* Do not expose `SpawnInTerminal` to user interface, avoid it
implementing `Serialize` and `Deserialize`
* Remove `NewCenterTask` action, extending `task::Spawn` interface
instead
* Do not require any extra unrelated parameters during task resolution,
instead, use task overrides on the resolved tasks on the modal side
* Add keybindings for opening the task modal in the
`RevealTarget::Center` mode

Release Notes:

- N/A

assets/keymaps/default-linux.json

@@ -426,7 +426,10 @@
       "ctrl-shift-r": "task::Rerun",
       "ctrl-alt-r": "task::Rerun",
       "alt-t": "task::Rerun",
-      "alt-shift-t": "task::Spawn"
+      "alt-shift-t": "task::Spawn",
+      "alt-shift-r": ["task::Spawn", { "reveal_target": "center" }]
+      // also possible to spawn tasks by name:
+      // "foo-bar": ["task::Spawn", { "task_name": "MyTask", "reveal_target": "dock" }]
     }
   },
   // Bindings from Sublime Text