Detect if password would be echoed, before entering password

[dfaerch]

Detect if the password would be visible, before we enter it. Maybe trigger some dialog to ask the user if they wish to paste anyway.

I've opened an issue wezterm/wezterm#6466 requesting for a lua function for detecting this.
If thats not possible, a workaround could be implemented, by:

• send a char (eg just a space or period)
• check if it is echoed back
• delete it again

Though this is doable, it does send chars to whatever program you have in foreground, if you try to send a password by accident. If user is in a program that reacts on single keypresses, eg. vi, then this can be annoying. So the preferred solution would be if something inside WezTerm can tell us if echo would happen.