htmlviewer-extension cleaned before packing.

This commit makes this package consistent with other packages.

packages/htmlviewer-extension/package.json

@@ -26,7 +26,7 @@
   "scripts": {
     "build": "tsc",
     "clean": "rimraf lib",
-    "prepack": "npm run clean && npm run build",
+    "prepublishOnly": "npm run build",
     "watch": "tsc -w"
   },
   "dependencies": {