UnitControl: Enable keyboard access (via tab) to unit select by default

[ItsJonQ]

This update ensures the unit select element within UnitControl is keyboard accessible by default.
The solution was to adjust the default setting for the component via the isUnitSelectTabbable prop.

(GIF above demonstrates keyboard tabbing through elements, including the unit select)

Resolves: #25703

How has this been tested?

Local Gutenberg

• npm run dev
• Add a Cover block
• Click on the Height control
• Press Tab. Ensure that you the unit select can be accessed.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• [n/a] My code has proper inline documentation.
• [n/a] I've included developer documentation if appropriate.
• [n/a] I've updated all React Native files affected by any refactorings/renamings in this PR.

[github-actions]

Size Change: -1 B

Total Size: 1.17 MB

Filename	Size	Change
build/components/index.js	168 kB	-1 B

ℹ️ View Unchanged

Filename	Size	Change
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build/annotations/index.js	3.52 kB	0 B
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build/blob/index.js	620 B	0 B
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build/block-directory/style-rtl.css	943 B	0 B
build/block-directory/style.css	942 B	0 B
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build/deprecated/index.js	772 B	0 B
build/dom-ready/index.js	568 B	0 B
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build/html-entities/index.js	621 B	0 B
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build/is-shallow-equal/index.js	709 B	0 B
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build/keycodes/index.js	1.85 kB	0 B
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build/list-reusable-blocks/style.css	476 B	0 B
build/media-utils/index.js	5.12 kB	0 B
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build/rich-text/index.js	13.5 kB	0 B
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build/warning/index.js	1.13 kB	0 B
build/wordcount/index.js	1.17 kB	0 B

_{compressed-size-action}

[mtias]

This makes sense to me as a default. Should it be disabled if there's a single unit available?

[ItsJonQ]

Should it be disabled if there's a single unit available?

It already does that 🎉 . It transforms from being a select to a div if there's only 1 label