Fix button functions behavior that differs from Playdate's

[AntonPetrov83]

1. Playdate functions playdate.buttonIsPressed(), buttonJustPressed, buttonJustReleased are case-insensitive.
   For example this snippet from the "2020":

		if playdate.buttonIsPressed("UP") then
			dy = -4
		elseif playdate.buttonIsPressed("DOWN") then
			dy = 4
		end
		if playdate.buttonIsPressed("LEFT") then
			dx = -4
		elseif playdate.buttonIsPressed("RIGHT") then
			dx = 4
		end

2. Constants kButtonUp, kButtonDown etc. must be integers and not strings.
3. Function playdate.getButtonState() must return the state of all buttons as three bit flag values (current, pressed, released).