feat(web): Add option to use system touch keyboard on Android/iOS

When using Keyman Web on a desktop system you can add an option to the page that allows you to select the system keyboard, allowing to type with the keyboard layout installed in the system.

It would be nice if that option would be available on mobile as well, allowing to use the system touch keyboard instead of the Keyman OSK.

A use case would be if a web page embeds Keyman Web and loads a Keyman keyboard allowing to type in a particular writing system. Advanced users might already have a different keyboard installed in their system that they would prefer to use.

This came out of EMDC 2022.