extensions-tool: Implement create command

extensions-tool: Implement create command

This implements more functionality of the existing tool and, as
'reload' is an unreliable feature that doesn't work more often
than not, the last bit that we will replicate.

The command follows the original for the most part, with the most
important difference being the installed template, which doesn't
provide any sample functionality and uses modern JS syntax.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/1234