Modifier-only triggers

[mitchellh]

There are multiple discussions where this would be an appropriate solution: #3734, #3775

We should allow modifier-only triggers, i.e.:

ctrl = text:hello
shift+ctrl = text:goodbye
ctrl>ctrl = toggle_quick_terminal

This is a tricky issue to do well. Currently, our keybindings process on key press. This is normal for any application out there. But modifiers make this unsafe because if you press ctrl+a for example then we'll get a ctrl followed by a. For this reason, I think modifier triggers should trigger on release. (and we should document this)

I think for multiple modifiers like shift+ctrl this will require some more complex accounting in our Surface too.

While doing this, we should look into sided modifier only triggers too, i.e.:

right_alt = text:hello

This issue doesn't need to enable sided modifiers i.e. right_alt+a. That's going to require separate work.

[Rotonen]

For the downstream work on splitting sided modifiers: typically one can make only the left alt do "meta sends esc" for compatibility with decades of muscle memory for irssi and various other more interactive console apps.

See: #967 (comment)