formatter(v, opts) {
      return v;
    }
Converts modelValue to formattedValue (formattedValue will be synced with
`._inputNode.value`)
For instance, a Date object to a localized date.
@param {*} v - modelValue: can be an Object, Number, String depending on the
input type(date, number, email etc)
@param {FormatOptions} opts
@returns {string} formattedValue