[Feature Request] VDatePicker: Distinguish out-of-range dates versus allowed dates

Problem to solve

When using the min and max props with allowed-dates, any date that isn't selectable, whether because it falls outside the range or because allowedDates(date) returns false, is rendered with the same "disabled" style. This makes it impossible to differentiate visually:

• Out-of-range (< min or > max)
• In-range but not allowed (within min/max, but filtered out by allowed-dates)

You can observe the allowed-dates example in the VDatePicker Vuetify documentation.

Proposed solution

Have v-date-picker apply two separate CSS classes (and document them) so consumers can target each case:

1. Dates outside min/max

• Class: v-date-picker-date--out-of-range

2. Dates inside the range but disallowed by allowed-dates

• Class: v-date-picker-date--not-allowed
  Optionally, expose corresponding named slots or props, but at a minimum, distinct classes will let us write custom styles:

.v-date-picker-date--out-of-range {
  opacity: 0.4;
}

.v-date-picker-date--not-allowed {
  text-decoration: line-through;
  color: rgba(255, 0, 0, 0.7);
}

Start by locating the VDatePicker implementation and the allowed-dates documentation example. Trace how min/max and allowed-dates currently mark dates, then add and document distinct classes for out-of-range and in-range disallowed dates; done means consumers can style the two cases separately.