Add entry for "hold to walk" in keybinds settings menu

So in case you didn't know, one can bind a key to `+walk`, i.e. `bind "LCTRL" "+walk"`. This is especially useful when using autosprint, as it allows you to go from sprinting to walking while holding `LCTRL` in this example.

I'm not sure how easy it is to add additional keybinds but if possible I think it makes sense to add an entry to set a key for `+walk` in the keybind options.