Fixes for the zoom behaviour and overlay height in iOS Safari

[eych]

Description

Address a few issues I've discovered when using the library in my project.

1. Implements the style fix for On iOS Safari overlay doesn't cover all of the screen's height because of height: 100svh style #433 as suggested by @rpearce
2. Allows to zoom on the already zoomed image with ctrl + wheel and pinch to zoom gestures on desktop
3. Triggers unzoom on touchmove instead of touchend. That is much closer to the behaviour on Medium (https://medium.design/image-zoom-on-medium-24d146fc0c20). Handling unzoom on touchend leads to nothing happening with the image while I touch-scroll until I lift my finger up.

Plus, renames master to main in CONTRIBUTING.md; otherwise, the instructions don't fully work.

Testing

1. Clone this project and go to the project directory
2. Run npm ci && npm start
3. Go to http://localhost:6006 (or equivalent local IP if you use a mobile device as suggested by Storybook on start)
4. See if the behaviour described above is working as intended

[rpearce]

Wow! This PR looks like a great addition. Thank you for taking the time to do this PR and update the docs; it really helps.

I need to test it across my devices, and I will do so as soon as I can (probably tonight).

[rpearce]

This feels like it works well, and you've now got me interested in making the mobile pinch-to-zoom experience better. Thanks for your contribution!