Create a new installing gif for Windows

Author: pimterry

package.json

@@ -49,6 +49,7 @@
         "exe": "httptoolkit.exe",
         "iconUrl": "https://httptoolkit.tech/favicon.ico",
         "setupIcon": "./src/icon.ico",
+        "loadingGif": "./src/installing.gif",
         "certificateFile": "./certificates/encrypted-win-cert.pfx"
       },
       "electronInstallerDMG": {