Provide current site name & identifier for build-time modifications

package.json

@@ -4,6 +4,7 @@
     "private": true,
     "scripts": {
         "build": "npm run build:popup && npm run build:extension",
+        "build:derpibooru": "cross-env SITE=derpibooru npm run build",
         "build:popup": "vite build",
         "build:extension": "node build-extension.js",
         "check": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json",