feat: Adds scheduler with wallclock timer (#2163)

### Summary

* Adds an event scheduler.
* Adds conveniences for hourly, daily, weekly, biweekly, monthly, ordinal monthly, and yearly recurrences

Example:

```cs
var tz = TimeZoneInfo.FindSystemTimeZoneById("America/New_York");

// Specify the time of the day, and the day of the week you want it to occur. Make sure it is translated into Utc.
// The next occurrence will be _after_ the specified date/time.
var scheduledEvent = EventScheduler.WeeklyAt(new DateTime(2025, 04, 26, 17, 00, 00), StartEvent, tz);

void StartEvent()
{
    World.Broadcast(0x30, false, "The event has started!");
}

Console.WriteLine("Event starts on {0}", scheduledEvent.NextOccurrence);
```

In this example, on _Saturday, May 3rd, 2025 @ 5pm ET_, the message "The event has started!" will be broadcasted.

Projects/Server/Main.cs

@@ -102,7 +102,8 @@ public static class Core
 
     private static long _tickCount;
 
-    private static DateTime _now;
+    // Make this available to unit tests for mocking
+    internal static DateTime _now;
 
     public static long TickCount => _tickCount;