Button: turn on the width setting by default in theme.json

[ramonjd]

What?

Turns on settings.blocks.core/button.dimensions.width in the bundled lib/theme.json so the Button block's Width control is available again in themes that don't opt into appearance tools.

Fixes the regression reported in #74242 (comment).

Why?

Before #74242 the Button block rendered its own WidthPanel, which was always visible. That PR replaced it with the shared dimensions.width block support, and the shared Dimensions panel only renders the control when the resolved theme.json setting is truthy

How?

One line in lib/theme.json:

 "core/button": {
 	"border": { "radius": true },
 	"dimensions": {
+		"width": true,
 		"dimensionSizes": [ ... ]
 	}
 }

Themes can still turn it off with settings.blocks."core/button".dimensions.width: false.

Testing Instructions

Test HTML

<!-- wp:buttons -->
<div class="wp-block-buttons"><!-- wp:button -->
<div class="wp-block-button"><a class="wp-block-button__link wp-element-button">Button</a></div>
<!-- /wp:button --></div>
<!-- /wp:buttons -->

1. Activate a classic theme that does not call add_theme_support( 'appearance-tools' ) for example Twenty Twenty-One.
2. Add a Button block and open the block inspector.
3. Before this change: there is no Width control in the Dimensions panel.
4. After this change: the Width control is there, with the 25/50/75/100% presets and a custom value input.
5. Repeat in Styles > Blocks > Button and confirm the Width control appears there too.
6. Switch to a block theme with appearanceTools: true (e.g. Twenty Twenty-Five) and confirm nothing changed there.

Before	After

[github-actions]

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Co-authored-by: ramonjd <ramonopoly@git.wordpress.org>
Co-authored-by: t-hamano <wildworks@git.wordpress.org>

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[t-hamano]

Thanks for the fix!

[github-actions]

I just cherry-picked this PR to the wp/7.1 branch to get it included in the next release: db5e69e