Form fields support the `maxlength` attribute, and a corresponding `tooLong` attribute on the `ValidityState` object -- an obviously useful feature.

However, it's common to require that passwords (and many other fields) conform to a minimum length, as well. Currently, this must be done with custom code or by an awkward `pattern` attribute.

I propose a `minlength` attribute, and a corresponding `ValidityState` attribute, `tooShort`.