fix(signature): enable trigger after show signature manually

[acidx27x]

Allow updating signature with trigger when turn on by keymap manually even when trigger is disabled globally like: signature = { enabled = true, trigger = { enabled = false, }, }.

Now when trigger is disabled, i need to invoke show_signature/hide_signature to update signature cursor position on typing. I think it is better to enable trigger update after call to show_signature manually. It looks like more expected behavior if user don`t want to trigger signature help automatically but still want to enable it by hand.