Add more customization for auto pan

[LouisBrunner]

Following the closure of #183 because of the change of the master branch to main, I recreate this PR as-is, original text follows:

Thanks a lot for this great project!

One of the issue (the only really) I ran into when integrating this in our product is around the auto-pan feature. I wanted to be able to customize the zone where auto-pan is triggered and the amount of space it shifts. So I added a bunch of properties:

• autoPanProps.length: length of the auto pan area on each side
• autoPanProps.width: length of the auto pan area on the left and right, overrides length
• autoPanProps.height: length of the auto pan area on the top and bottom, overrides length
• autoPanProps.delta: amount of pixel to shift when auto panning
• autoPanProps.easing: easing function which must return the amount of pixel to shift given t [0, 1], 0 being the start of the panning area, 1 being the edge of the viewer, overrides delta

The default behavior is still the same and corresponds to autoPanProps={{length: 20, delta: 2}}.

Another change I included was around the action used by auto-pan, changing it from ACTION_PAN to ACTION_AUTO_PAN, which is a breaking-change, but I needed to be able to distinguish regular panning from auto-panning (as our SVG is draggable as well and I want to disable regular panning when that is going but not auto-panning).

Last but not least, I just added a prepare script to allow to use the repository with npm link or file:// in package.json.

Don't hesitate to give me a lot of feedback about any issue with these changes (especially ACTION_AUTO_PAN) as I am really keen to get these behaviors in! :)

[sonarcloud]

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