fix(input): use ValidityState to determine validity

In browsers where HTML5 constraint validation is (partially) implemented, an invalid number
entered into an input[type=number] (for example) input element would be visible to the
script context as the empty string. When the required or ngRequired attributes are not used,
this results in the invalid state of the input being ignored and considered valid.

To address this, a validator which considers the state of the HTML5 ValidityState object is
used when available.

Closes #4293
Closes #2144
Closes #4857
Closes #5120
Closes #4945
Closes #5500
Closes #5944