Fixes the issue where the "!important" rule is not applied in the cas…

…e of an UnknownValue (AllowInvalidValues = true).

In the following CSS, the value is recognized as unknown, but important is ignored.

.moderntable > .moderntable-caption {
  width: min-content !important;
}

src/ExCSS/Parser/StylesheetComposer.cs

@@ -657,6 +657,7 @@ public Property CreateDeclarationWith(Func<string, Property> createProperty, ref
 
                             property = new UnknownProperty(propertyName);
                             property.TrySetValue(value);
+                            property.IsImportant = important;
                             _nodes.Push(property);
                         }
                     }