Use fromCodePoint to convert high value unicode entities (#243)

* Use fromCodePoint to convert high value unicode entities

* Include polyfill for String.fromCodePoint

* copy and adapt fromCodePoint into String.fromcodepoint

In order to avoid modifying String as the polyfill does, I've copied
the source from the polyfill and adapted it return the polyfill
function if the native version does not exist. Once support for node
versions that lack fromCodePoint is dropped, this polyfill can be
removed.

* move license notice to top of file

test/fixtures/jsx/basic/entity/expected.json

@@ -136,7 +136,7 @@
                 }
               },
               "extra": null,
-              "value": ""
+              "value": "💩"
             }
           ]
         }