feat(DurationUnit): add Weekend duration unit

- Introduces a new duration unit representing one weekend (2 days).
- Enhances flexibility in duration calculations for scheduling and time management features.

Author: winromulus

src/ES.FX/Primitives/DurationUnit.cs

@@ -95,6 +95,11 @@ public enum DurationUnit
     /// </summary>
     Week,
 
+    /// <summary>
+    ///     One weekend (2 days).
+    /// </summary>
+    Weekend,
+
     /// <summary>
     ///     One calendar month (28–31 days).
     /// </summary>