fix: disabling start for middle mouse button

There are 3 possible values for `event.button`:
- 0: left mouse button
- 1: middle mouse button
- 2: right mouse button

We should not start dragging if either middle or right mouse buttons are clicked

Author: cgood92

src/SortableContainer/index.js

@@ -36,6 +36,8 @@ import {
   defaultKeyCodes,
 } from './props';
 
+const LEFT_MOUSE_BUTTON = 0;
+
 export const SortableContext = React.createContext({
   manager: {},
 });
@@ -124,7 +126,7 @@ export default function sortableContainer(
     handleStart = (event) => {
       const {distance, shouldCancelStart} = this.props;
 
-      if (event.button === 2 || shouldCancelStart(event)) {
+      if (event.button !== LEFT_MOUSE_BUTTON || shouldCancelStart(event)) {
         return;
       }