Add Date Schema Type

Support validating date fields using a dedicated `validateDateValue` function.

This should validate `JavaScript Date objects` as well as `ISO 8601 date strings`, and return a clear validation error for invalid dates.

Valid values
```typescript
new Date("2026-07-07")
"2026-07-07T10:30:00Z"
```

Invalid values
```typescript
"not-a-date"
"2026-13-40"
12345
```