feat: change type to 'theme'

Signed-off-by: ItsNeil17 <neil@willofsteel.me>

Author: ItsNeil17

composer.json

@@ -4,7 +4,7 @@
     "keywords": [
         "flarum"
     ],
-    "type": "flarum-extension",
+    "type": "theme",
     "license": "MIT",
     "require": {
         "flarum/core": "^1.8"