Add a factory constructor for customizing snap behavior

[fujidaiti]

If we want to add an additional snap position of the viewport offset, we would typically write code as follows, but it's a bit verbose.

const peekOffset = ViewportOffset.fractional(0.5);
controller = ExprollablePageController(
  initialViewportOffset: peekOffset,
  maxViewportOffset: peekOffset,
  snapViewportOffsets: [
    ViewportOffset.expanded,
    ViewportOffset.shrunk,
    peekOffset,
  ],
);

It would be nice to have a factory constructor of ExprollablePageController for simplicity like:

const peekOffset = ViewportOffset.fractional(0.5);
controller = ExprollablePageController.withAdditionalSnapOffsets(
  initialViewportOffset: peekOffset,
  additionalSnapViewportOffsets: [ peekOffset ],
);

[fujidaiti]

Close this issue as it has been implemented and will be included in the next release.