Feat: add shift scroll and global scroll

[Zyie]

• Add globalScroll option, which when set to false only scroll when he pointer is over the top of the scroll box
• Add shiftScroll option, which when the scrollbox is horizontal forces the user to use the shift + scroll to move the scrollbox, better emulating how scroll boxes work on the browser

[TimAstier]

That will be useful!

@Zyie Maybe a related use-case:
Is there a way to avoid triggering other (global?) wheel events when scrolling within a scrollbox?
I had to add some custom events to get this type of behavior.

Specifically, scrolling in a scrollbox would also trigger the wheel event of pixi-viewport (meaning, zoom in / out).

So I had to do something like:

scrollbox.on('pointerenter', () => {
      this._isScrollboxActive = true;
      this.game.engine.viewport.pauseWheel();
    });
    scrollbox.on('pointerleave', () => {
      this._isScrollboxActive = false;
      this.game.engine.viewport.resumeWheel();
    });

// A little bit hacky, cannot figure out how to avoid scrolling on any global wheel event
      if (this._isScrollboxActive === false) {
        this._scrollbox.scrollTop();
      }

As I understand, the fix from this PR will remove the need for 2).