Add static Runnables (#8009)

Part of #7108

This PR includes just the static runnables part. We went with **not**
having a dedicated panel for runnables.
This is just a 1st PR out of N, as we want to start exploring the
dynamic runnables front. Still, all that work is going to happen once
this gets merged.

Release Notes:

- Added initial, static Runnables support to Zed. Such runnables are defined in
`runnables.json` file (accessible via `zed: open runnables` action) and
they can be spawned with `runnables: spawn` action.

---------

Co-authored-by: Kirill Bulatov <kirill@zed.dev>
Co-authored-by: Pitor <pitor@zed.dev>
Co-authored-by: Beniamin <beniamin@zagan.be>

assets/settings/initial_runnables.json

@@ -0,0 +1,19 @@
+// Static runnables configuration.
+//
+// Example:
+// {
+//     "label": "human-readable label for UI",
+//     "command": "bash",
+//     // rest of the parameters are optional
+//     "args": ["-c", "for i in {1..10}; do echo \"Second $i\"; sleep 1; done"],
+//     // Env overrides for the command, will be appended to the terminal's environment from the settings.
+//     "env": {"foo": "bar"},
+//     // Current working directory to spawn the command into, defaults to current project root.
+//     "cwd": "/path/to/working/directory",
+//     // Whether to use a new terminal tab or reuse the existing one to spawn the process, defaults to `false`.
+//     "use_new_terminal": false,
+//     // Whether to allow multiple instances of the same runnable to be run, or rather wait for the existing ones to finish, defaults to `false`.
+//     "allow_concurrent_runs": false,
+// },
+//
+{}