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>

crates/zed/src/main.rs

@@ -244,6 +244,7 @@ fn main() {
         outline::init(cx);
         project_symbols::init(cx);
         project_panel::init(Assets, cx);
+        runnables_ui::init(cx);
         channel::init(&client, user_store.clone(), cx);
         search::init(cx);
         semantic_index::init(fs.clone(), http.clone(), languages.clone(), cx);