reason-react-native: Add AppRegistry.registerHeadlessTask (#560)

Author: cknitt

reason-react-native/src/apis/AppRegistry.md

@@ -9,6 +9,11 @@ wip: true
 external registerComponent: (string, unit => React.component('a)) => unit =
  "";
 
+type task('data) = 'data => Js.Promise.t(unit);
+
+[@bs.module "react-native"] [@bs.scope "AppRegistry"]
+external registerHeadlessTask: (string, unit => task('data)) => unit = "";
+
 // react-native-web
 type app = {
  .

reason-react-native/src/apis/AppRegistry.re

@@ -2,6 +2,11 @@
 external registerComponent: (string, unit => React.component('a)) => unit =
  "";
 
+type task('data) = 'data => Js.Promise.t(unit);
+
+[@bs.module "react-native"] [@bs.scope "AppRegistry"]
+external registerHeadlessTask: (string, unit => task('data)) => unit = "";
+
 // react-native-web
 type app = {
  .