✨Added required "ghost-theme" keyword

- We're going to start showing this as a warning in gscan
- The purpose is to give us a way to detect if a package is a valid Ghost theme

package.json

@@ -29,7 +29,8 @@
     },
     "keywords": [
         "ghost",
-        "theme"
+        "theme",
+        "ghost-theme"
     ],
     "repository": {
         "type": "git",