Page gap support

[hvesuk]

This PR adds a new pageGap parameter to PhotoViewGallery.

Behavior:

• pages stay full-screen when the gallery is settled
• a configurable gap is shown only while swiping between pages
• defaults to 0.0, so the change is backward compatible

Implementation:

• added pageGap to both PhotoViewGallery constructors
• updated the internal page rendering to shift pages dynamically during scroll
• left existing gallery behavior unchanged when pageGap is not set

[hvesuk]

Hi @renancaraujo, tagging you as the most active contributor to this repo

I made a page gap support. Not sure this is best way possible, but it works for me

Let me know if any corrections needed