[ACCESSIBILITY] enable the timeout to be optionally decoupled from `timoutlen`

[Corey-Keller]

As a disabled user (can only use 1 hand, plus other issues) I prefer to have high values (currently 3000) for my timeoutlen so that it doesn't cancel if it takes a few seconds for me to find the right key. The problem is then that the which-key popup is then delayed that same long 3 seconds before displaying.

This is similar to #163, but different in that it sets a global default value, as opposed to the individual overrides of the default like in #163.