CustomSelectControlV2: keep item checkmark top aligned

[ciampo]

What?

Part of #55023
Extracted from #63139

Why?

Design feedback

How?

By using flexbox's alignment options + adding a small margin to preserve the correct vertical alignment with the first line of item text.

Testing Instructions

1. Apply the following diff

diff --git a/packages/components/src/custom-select-control/stories/index.story.tsx b/packages/components/src/custom-select-control/stories/index.story.tsx
index 8ff9a023c5..9ebf38ab3d 100644
--- a/packages/components/src/custom-select-control/stories/index.story.tsx
+++ b/packages/components/src/custom-select-control/stories/index.story.tsx
@@ -111,7 +111,8 @@ WithHints.args = {
 		{
 			key: 'medium',
 			name: 'Medium',
-			__experimentalHint: '250x250',
+			__experimentalHint:
+				'A very long hint just to test the checkmark alignment (250x250)',
 		},
 		{
 			key: 'large',

2. Open the CustomSelectControlV2 Legacy "With Hints" example
3. Select the "Medium" item
4. Open the select popover, make sure that the checkmark is correctly top aligned
5. Select other items on this and other examples, make sure that the checkmark is center aligned with the item text when the option item fits on one row

Screenshots or screencast

Scenario	Before (trunk)	After (this PR)
Selected item on one line
Selected item on multiple lines

[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]

Tests as advertised and code looks good 👍 🚀

I'm starting to prefer this solution design-wise, too.

[github-actions]

Flaky tests detected in 43cb26f.
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/9838189617
📝 Reported issues:

• [Flaky Test] should allow user to add and remove multiple local font files #62862 in /test/e2e/specs/site-editor/font-library.spec.js