feat(custom-dnd-backend): Add props to override react-dnd backend

[robbertkl]

My last one for now, I promise 😉

I realise this is a bit of a power user feature, but I was in need of using the react-dnd's TouchBackend instead of MultiBackend+HTML5toTouch. The reason is I'm using sound effects and the HTML5 drags for some reason don't count as user interaction, so modern browsers will disallow audio playing if it's not triggered by user interaction. TouchBackend (with mouse enabled) works perfectly fine for me, even though it has a note about being buggy and it is recognized as user interaction.

Anyway, only using TouchBackend might not be for everyone, so for this PR I decided to add a prop (or 2 actually) to be able to swap out the backend for a different one.

Usage:

import { Chessboard } from 'react-chessboard';
import { TouchBackend } from 'react-dnd-touch-backend';

...

<Chessboard
  ...
  customDndBackend={TouchBackend}
  customDndBackendOptions={{ enableMouseEvents: true }}
/>

Of course, leaving out these props will keep the existing behavior of using MultiBackend+ HTML5toTouch.

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