feat(input[range]): support step

Step support works like min / max, but with the following caveat.
Currently, only Firefox fully implements the spec. Other browsers
(Chrome, Safari, Edge) have issues when the step value changes
after the input has been changed. They do not adjust the input value
to a valid value, but instead set the stepMismatch validity state.

Angular will take this validity state, and forward it as the ngModel
"step" error. Adjusting the error ourselves would add too much code,
as the logic is quite involved.