Add horizontal scrolling and support for nesting ScrollingPanes

[kaycodes13]

Summary of Changes

This adds:

• Support for horizontal scrolling
• Support for nesting ScrollingPanes within other ScrollingPanes

(no version bump due to 4 open PRs at time of writing)

Checklist

• No change is too small for a release, so pick one:
  • I have updated the package version following semantic versioning
  • There is another change actively WIP that will update the version
  • This PR does not update user-facing code/config
  • I'm not sure how to set the version and would like the reviewer's help

[kaycodes13]

Before I mark this as ready for review, I was hoping I could get a bit of help with an odd navigation order issue.

On the first page of the "Scrolling Pane Tests" menu I built, The content is structured:

GridGroup (2 columns)
    ScrollingPane
        VerticalGroup
            TextButton x 15
    ScrollingPane
        VerticalGroup
            TextButton x 15

For the most part this is working as expected, except for some reason, Navigating left and right between the two scroll panes always focuses the middle button in the group - button 7 - regardless of which button you're navigating from or what the state of either scroll pane is.

I don't have a good grasp of how the navigation is set up, but that behaviour doesn't seem correct to me. I'd expect to either navigate to what is visually the closest selectable to the side of the one I was on is, or maybe to go to the top of the next pane over.

Would anyone mind taking a look and trying to sort out what's going on there and if it's correct or not? 😅

[kaycodes13]

After discussing with Flibber, it seems like that issue is related to the VerticalGroup code, not the ScrollingPane, so this is good for review now.

[flibber-hk]

As with the other PR, feel free to make some noise in Discord if you want a release before the other extant PRs are gotten to