Add BoxShadow CSSProp

When I tried to write inline style for box-shadow css property, I found out that it is missing in CSSProp discriminated union. So here is a fix.

Author: worldbeater

src/Fable.React/Fable.Helpers.React.fs

@@ -380,6 +380,7 @@ module Props =
         | BoxLineProgression of obj
         | BoxLines of obj
         | BoxOrdinalGroup of obj
+        | BoxShadow of obj
         | BreakAfter of obj
         | BreakBefore of obj
         | BreakInside of obj