Combobox Control: Add placeholder attribute

[zaguiini]

What?

Title.

Why?

Increase UX of the Combobox control by making possible to specify a placeholder for it.

How?

Accepts a placeholder prop and passes it to the <TokenInput /> element.

Testing Instructions

Open Storybook and pick the Components > ComboboxControl component. You should see the placeholder applied.

Screenshots or screencast

[github-actions]

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Co-authored-by: zaguiini <zaguiini@git.wordpress.org>
Co-authored-by: tyxla <tyxla@git.wordpress.org>
Co-authored-by: mirka <0mirka00@git.wordpress.org>

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[tyxla]

This will address part of #37011

[tyxla]

This looks good overall 👍 Thanks @zaguiini!

I've left a couple of minor suggestions.

Also the PR could use a CHANGELOG entry.

[zaguiini]

Thanks for the help @tyxla @mirka this is ready for another review!

[mirka]

Looks good, thanks! 🚀