ItemGroup: Improve stories to default to bordered and separated

[tyxla]

What?

Improves the ItemGroup stories to have bordered and separated styles.

Why?

Because this is the most common usage and it demonstrates better how the component is composed.

How?

Enabling isBordered and isSeparated for all stories, and inverting the "WithBorder" story to be a "WithoutBorder" one.

Testing Instructions

Spin up a Storybook instance and test ItemGroup - go through all stories and ensure bordered and separated is now the default, and the non-bordered story works correctly.

Testing Instructions for Keyboard

Same

Screenshots or screencast

Before	After

[github-actions]

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Co-authored-by: tyxla <[email protected]>
Co-authored-by: ciampo <[email protected]>

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[ciampo]

LGTM 🚀

I wonder if we should also change the default snippet in the JSDoc and even add a little sentence in that snippet about usually needing to set the two properties to true for the most typical visual styles.

[ciampo]

Nit: for consistency with how we usually write the stories:

Suggested change

	isBordered: true,
	isSeparated: true,
	...Default.args,

[tyxla]

Sounds good, done in 5e26b6f

[ciampo]

Same as above

[tyxla]

Addressed as part of 5e26b6f

[tyxla]

I wonder if we should also change the default snippet in the JSDoc and even add a little sentence in that snippet about usually needing to set the two properties to true for the most typical visual styles.

This is exactly why I wanted to introduce this as a breaking change. If we need to explain this behavior, then it just doesn't make sense and needs to be changed.

If y'all agree this is the best course of action, I'm happy to follow-up and address all ItemGroup usage to invert those props.

[github-actions]

Flaky tests detected in 5e26b6f.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/11382763697
📝 Reported issues:

• [Flaky Test] should move focus to the block #38832 in /test/e2e/specs/widgets/customizing-widgets.spec.js