Add callable workflow for extension repositories (#43082)

This starts the work on a workflow that can be invoked in extension CI
to test changes on extension repositories.

Release Notes:

- N/A

---------

Co-authored-by: Agus Zubiaga <agus@zed.dev>
Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

.github/workflows/danger.yml

@@ -12,7 +12,7 @@ on:
     - main
 jobs:
   danger:
-    if: github.repository_owner == 'zed-industries'
+    if: (github.repository_owner == 'zed-industries' || github.repository_owner == 'zed-extensions')
     runs-on: namespace-profile-2x4-ubuntu-2404
     steps:
     - name: steps::checkout_repo