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/Cargo.toml

@@ -88,6 +88,8 @@ release_channel.workspace = true
 rope.workspace = true
 rpc.workspace = true
 rsa = "0.4"
+runnable.workspace = true
+runnables_ui.workspace = true
 rust-embed.workspace = true
 schemars.workspace = true
 search.workspace = true