[Feature]: Support "termux:" scheme when temux-scheme-opener script is created.

[c6401]

Feature description

To register "termux:" scheme, which will be handled by bin/termux-scheme-opener shell script, but only in case if user created one. If the script is not present, there probably should be a dialog telling so. The script should receive the passed URL with scheme (and possibly the sender app name).

Security:
Disabled by default, till termux-scheme-opener script is created, users can deploy their own ways to restrict this action such as passing some token in the url "termux://myaction?token=secret" to allow the command or creating a termux shell question hot to act on open.

Reasoning:
It will allow creating mobile browser userscripts or bookmarklets to send some actions from the browser to termux.

Additional information

Registering some custom scheme to run handler scripts is a popular automation technique on desktop operations systems. Example:
https://github.com/alphapapa/org-protocol-capture-html