json: Register tasks on Rust side and not via tasks.json (#12345)

/cc @RemcoSmitsDev new task indicators weren't showing for me in JSON
files.
`tasks.json` of native grammars is not being read by anything by
default, so we tend to register tasks as Rust structs, foregoing the
deserialization step. This doesn't apply to tasks registered in
extensions, which have to have tasks.json.

Release Notes:

- N/A

crates/languages/src/json/tasks.json

@@ -1,14 +0,0 @@
-[
-  {
-    "label": "package script $ZED_CUSTOM_script",
-    "command": "npm run",
-    "args": ["$ZED_CUSTOM_script"],
-    "tags": ["package-script"]
-  },
-  {
-    "label": "composer script $ZED_CUSTOM_script",
-    "command": "composer",
-    "args": ["$ZED_CUSTOM_script"],
-    "tags": ["composer-script"]
-  }
-]