Add option to specify a URL in the manual extension (#1479)

* Add option to specify a URL in the manual extension

* Fix error when option parameter isn't provided
The uglify-js dependency had to be updated to support the optional chaining operator

* Remove the possibility to pass a function for the custom URL
This added weight unnecessarily, as the users can simply call the function themselves before triggering the event.

* Change optional chaining to ternary for improved browser support

* Revert package-lock.json to version 3.9.4 of uglify-js
It was previously updated to support optional chaining; reverting it as a reminder that this is the current browser support level that is accepted by Plausible.