Block switch: ensure hover is reset when style changed

[johngodley]

The block style picker is used by a block's toolbar to allow the style to be changed. As you hover over a style it sends a onHoverClassName event to BlockSwitcher indicating which style is being hovered over, which is then used to show a preview.

When you click on a style the BlockSwitcher is hidden but retains its last state, including the hover style.

If you show the BlockSwitcher again it will appear with a hover style and preview already selected

This PR sends a onHoverClassName event when a style is picked, ensuring the BlockSwitcher component is reset.

Fixes #12225

How has this been tested?

1. Create a button block and enter some details
2. Click the block style icon in the block toolbar (not the sidebar)
3. Pick a style
4. Click the block style icon again and verify that the preview is not shown
5. Verify that the block style picker in the sidebar functions as expected - nothing should change here as it doesnt show a preview

Types of changes

Non-breaking change to <BlockStyles/>

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.

[youknowriad]

LGTM 👍 Thanks for the fix.