Update to Manifest V3

This should allow the add-on to work again.

Recently, Ivanti (the company behind the VPN) changed the web UI to a React app. This broke the extension.

With this new version of the extension, the additional UI not only integrates better with the one from Ivanti - it also adds a "Copy" button for easy copy-pasting of the `openconnect` command.

This version adds both "author" and "developer" details to the Manifest file so that the source code can be accessed right from the add-on's page.