Deprecate the ext.Ajax extension.

Recommend to use `dom.fetch` instead, whose API is basically just
as good as `Ajax` was.

Author: sjrd

src/main/scala/org/scalajs/dom/ext/Extensions.scala

@@ -16,6 +16,7 @@ import org.scalajs.dom.{Blob, FormData}
  * Thrown when `Ajax.get` or `Ajax.post` receives a non-20X response code.
  * Contains the XMLHttpRequest that resulted in that response
  */
+@deprecated("use the dom.fetch API instead", "2.0.0")
 case class AjaxException(xhr: dom.XMLHttpRequest) extends Exception {
   def isTimeout = xhr.status == 0 && xhr.readyState == 4
 }
@@ -24,6 +25,7 @@ case class AjaxException(xhr: dom.XMLHttpRequest) extends Exception {
  * Wraps an XMLHttpRequest to provide an easy one-line way of making
  * an Ajax call, returning a Future.
  */
+@deprecated("use the dom.fetch API instead", "2.0.0")
 object Ajax {
 
   /**