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

Cargo.lock

@@ -15603,7 +15603,6 @@ dependencies = [
  "ui",
  "util",
  "uuid",
- "zed_actions",
 ]
 
 [[package]]
@@ -16108,6 +16107,7 @@ name = "zed_actions"
 version = "0.1.0"
 dependencies = [
  "gpui",
+ "schemars",
  "serde",
 ]