Add option to lock axis while dragging

[itslenny]

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[ X ] feature request
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Current behavior

There is no way to lock axis while dragging

Expected behavior

It would be nice to be able to lock the x / y axis for lists where the items should only be dragged in one or the other

Someone submitted this to dragula's main repo a long time ago, but since that project is dead it was never merged. I'm happy to port that PR over and add the implementation to ngx-dnd too, but I wanted to create an issue first to make sure this feature is acceptable to the maintainer(s).

[Hypercubed]

I'd be willing to look at a PR. Note that I want to drop dragula ASAP. So some of that work will need to be redone when we do....