Fix type for StyleSheet.compose()

Summary:
This was flagged when typechecking the existing examples. The current type enforces that both stylesheets overlap, meaning composed stylesheets with distinct props cause a typechecking error. This changes the signature so that each style can be different, and the intersection type of the two is returned.

Changelog:
[General][Fixed] - Fix type for `StyleSheet.compose()`

Reviewed By: christophpurrer

Differential Revision: D42008355

fbshipit-source-id: 238971a61b387c09be001d5df50fe3db70df566f

Author: NickGerleman

Libraries/StyleSheet/StyleSheet.d.ts

@@ -87,10 +87,10 @@ export namespace StyleSheet {
    * an array, saving allocations and maintaining reference equality for
    * PureComponent checks.
    */
-  export function compose<T>(
+  export function compose<T, U>(
     style1: StyleProp<T> | Array<StyleProp<T>>,
-    style2: StyleProp<T> | Array<StyleProp<T>>,
-  ): StyleProp<T>;
+    style2: StyleProp<U> | Array<StyleProp<U>>,
+  ): StyleProp<T & U>;
 
   /**
    * WARNING: EXPERIMENTAL. Breaking changes will probably happen a lot and will