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

crates/zed_actions/src/lib.rs

@@ -1,5 +1,6 @@
 use gpui::{actions, impl_actions};
-use serde::Deserialize;
+use schemars::JsonSchema;
+use serde::{Deserialize, Serialize};
 
 // If the zed binary doesn't use anything in this crate, it will be optimized away
 // and the actions won't initialize. So we just provide an empty initialization function
@@ -90,33 +91,39 @@ pub struct OpenRecent {
 gpui::impl_actions!(projects, [OpenRecent]);
 gpui::actions!(projects, [OpenRemote]);
 
-#[derive(PartialEq, Eq, Clone, Copy, Deserialize, Default, Debug)]
+/// Where to spawn the task in the UI.
+#[derive(Default, Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize, JsonSchema)]
 #[serde(rename_all = "snake_case")]
-pub enum TaskSpawnTarget {
+pub enum RevealTarget {
+    /// In the central pane group, "main" editor area.
     Center,
+    /// In the terminal dock, "regular" terminal items' place.
     #[default]
     Dock,
 }
 
 /// Spawn a task with name or open tasks modal
-#[derive(PartialEq, Clone, Deserialize, Default)]
-pub struct Spawn {
-    #[serde(default)]
-    /// Name of the task to spawn.
-    /// If it is not set, a modal with a list of available tasks is opened instead.
-    /// Defaults to None.
-    pub task_name: Option<String>,
-    /// Which part of the UI the task should be spawned in.
-    /// Defaults to Dock.
-    #[serde(default)]
-    pub target: Option<TaskSpawnTarget>,
+#[derive(Debug, PartialEq, Clone, Deserialize)]
+#[serde(untagged)]
+pub enum Spawn {
+    /// Spawns a task by the name given.
+    ByName {
+        task_name: String,
+        #[serde(default)]
+        reveal_target: Option<RevealTarget>,
+    },
+    /// Spawns a task via modal's selection.
+    ViaModal {
+        /// Selected task's `reveal_target` property override.
+        #[serde(default)]
+        reveal_target: Option<RevealTarget>,
+    },
 }
 
 impl Spawn {
     pub fn modal() -> Self {
-        Self {
-            task_name: None,
-            target: None,
+        Self::ViaModal {
+            reveal_target: None,
         }
     }
 }