add ability to create custom page up/down step size (see https://github.com/phetsims/molecules-and-light/issues/143), customgetter & setter for _pageKeyboardStep
add ability to create custom page up/down step size (see https://github.com/phetsims/molecules-and-light/issues/143), customgetter & setter for _pageKeyboardStep

phetsims/sun,phetsims/sun,phetsims/sun