Support for drag and drop synthetic events

[evbo]

I can probably submit a PR eventually, but just wanted to capture this in case someone has a fix already!

drag and drop is of course a stunner HTML5 delivered and my understanding is you need access to types similar to ClipBoard events, e.g.:

.dataTransfer.files

For reference, I'm not seeing a drag/drop "dataTransfer" js.native for MouseEvent:

https://github.com/shadaj/slinky/blob/b1e7e021802d1503684f6db40aca9ed4ea80320c/web/src/main/scala/slinky/web/SyntheticMouseEvent.scala

So these should be added to MouseEvent

[evbo]

@shadaj would you prefer we just piggy back on the existing MouseEvent or would you like to break things up by context, such as creating a separate DragEvent as shown here?:
https://www.programcreek.com/scala/org.scalajs.dom.raw.Event

[shadaj]

I guess the primary authority on this is React's synthetic events. I don't see a specific type for drag events, so using MouseEvent would probably be the way to go.