Editor: Fix visibility of sibling inserter on tab focus

[aduth]

Likely regressed in #11018

This pull request seeks to resolve an issue (regression) where tab-focusing the sibling inserter would not cause it to become visible. It would still be operable by space-activation. With these changes, the button itself is now once-again visible.

Before	After

Implementation notes:

The issue is that in adapting the sibling inserter to use the Inserter in #11018, the .is-visible modifier applying opacity was not correctly adapted to .editor-block-list__insertion-point-inserter.

It's unclear whether we need two separate classes each applying opacity.

Testing instructions:

Verify that, via tab navigation, the sibling inserter button is made visible once focused.

[jorgefilipecosta]

It worked great in my tests 👍
It looks like there may be a small delay/animation before the button appears though.

But this is for sure not a blocker.