imp - Check for DTSTART if there is no METHOD

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According to the vCalendar 2.0 specifications, if there is no METHOD, then DTSTART is required.

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Type: imp
Breaking: False
Doc Required: False
Backport Required: False
Part: 1/1

VisualCard.Calendar/Parsers/VCalendarParser.cs

@@ -282,6 +282,10 @@ internal void ValidateCalendar(Parts.Calendar calendar)
                 calendar.CalendarVersion.Major == 2 ?
                 [VCalendarConstants._uidSpecifier, VCalendarConstants._dateStampSpecifier] : [];
             string[] expectedTodoFields = expectedEventFields;
+            expectedEventFields =
+                calendar.CalendarVersion.Major == 2 && calendar.GetString(CalendarStringsEnum.Method).Length == 0 ?
+                [VCalendarConstants._dateStartSpecifier, .. expectedEventFields] :
+                expectedEventFields;
             foreach (var eventInfo in calendar.events)
             {
                 if (!ValidateComponent(ref expectedEventFields, out string[] actualEventFields, eventInfo))