Added a toggle button in options (not bound by default).

Toggling works with the default keybind (i.e. if you are toggled to prone, the hold button will make you stand up while it is held).

The toggle is tied to your game client, so there are a few minor issues that I will probably (maybe if it bothers people) fix at some point:

• If you change save games with the toggle on, you will be prone in the new world (although I'm not sure how that interacts with changing servers).
• If you quit while prone is toggled on, next time you join you will be standing up (unless the toggle is still on).

Requires Forge 28.1.0 or later (MC 1.14.4)