Added as property to DOM props (#603)

* Added `as` property to DOM props

* Added the as prop to the props type

Author: Corné Dorrestijn

src/ReactDOM.re

@@ -207,6 +207,8 @@ module Props = {
     [@bs.optional]
     itemType: string, /* uri */
     /* tag-specific html attributes */
+    [@bs.optional] [@bs.as "as"]
+    as_: string,
     [@bs.optional]
     accept: string,
     [@bs.optional]
@@ -1234,6 +1236,8 @@ module Props = {
     [@bs.optional]
     itemType: string, /* uri */
     /* tag-specific html attributes */
+    [@bs.optional] [@bs.as "as"]
+    as_: string,
     [@bs.optional]
     accept: string,
     [@bs.optional]