Shadow Panel: Add reset button

[yogeshbhutkar]

What, Why and How?

Closes #68973

This PR adds a reset button to the Shadow Panel, which becomes visible on hover or focus.

Testing Instructions

1. Navigate to the post edit page.
2. Create a Group block and try changing the default shadow.
3. Observe the presence of the new reset button, which appears on hover.
4. Try pressing the reset button and confirm the shadow gets reset.

Screencast

shadow-demo.mov

[github-actions]

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Co-authored-by: t-hamano <wildworks@git.wordpress.org>
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[t-hamano]

This PR works well, but from an accessibility perspective, we need to check the following points in particular:

• It must be operable only via keyboard.
• The focus must never be lost.

That is, the focus must not be lost when the reset button is pressed. See this implementation for reference:

gutenberg/packages/block-editor/src/components/colors-gradients/dropdown.js

Lines 136 to 137 in 0feca71

	// Return focus to parent button
	colorButtonRef.current?.focus();

[yogeshbhutkar]

Do I also extract the renderToggle logic into a generalized reusable component within this PR and use it within shadow-panel component?

Ref: #68973 (comment)

[t-hamano]

Personally, I find it easier to implement reset buttons in all the places and then extract them into reusable components.

[t-hamano]

LGTM! I think everything is working as expected.

797ca976ae6f4b0c08eea2cb8bd3db0c.mp4