feat: add the ability to toggle scrollbar visibility

[BeksOmega]

The basics

• I branched from develop
• My pull request is against develop
• My code follows the style guide
• I ran npm run format and npm run lint

The details

Resolves

Fixes #6971

Proposed Changes

Adds a method to toggle the visibility of a scrollbar pair.

Reason for Changes

App inventor needs this to reduce clutter when opening flyouts.

Test Coverage

Manually tested that setting the scrollbar visibility hides the scrollbars.

Documentation

N/A

Additional Information

I thought about solving this in a bunch of different ways, but the only one that ended up working was adding the internal setVisible method.

Removing the this.pair check alone doesn't work because external devs can access the individual scrollbars and try to mess with them, so we will run into the corner-error that the comment references. And we can't fix that by by telling the scrollbar pair to update itself when an individual scrollbar gets hidden, because having a reference to the scrollbar pair would cause circular dependencies.