CustomSelectControlV2: collapse checkmark space when unchecked

[ciampo]

What?

Part of #55023

Extracted from #63179

Allow checkmark wrapper to collapse when not shown

Why?

So that the item's content can go full width in most scenarios, like it does already in the legacy V1 implementation

How?

By setting the font-size to 0 when necessary.

Testing Instructions

• In Storybook, visit the CustomSelectControlV2 legacy "with long labels" example
• Open the select popover, and resize the page until the text needs to wrap on two or more lines.
• Notice how the text goes all the way to the end of the popover (since the checkmark is collapsed)
• Select that item, and notice how the checkmark appears, causing the text to take less space
• Make sure this is the same behavior of V1
• Visit the "With Hints" example, and make sure that the checkmark wrapper never collapses, even for items that are not checked

Screenshots or screencast

Legacy V1	V2 adapter - before (trunk)	V2 adapted - after (this PR)

[github-actions]

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Co-authored-by: ciampo <mciampini@git.wordpress.org>
Co-authored-by: tyxla <tyxla@git.wordpress.org>

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[tyxla]

LGTM and tests well: