Fix CustomSelectControl component dropdown arrow spacing

[getdave]

This PR fixes an issue identified in #18869 (comment) whereby if the selected option in a CustomSelectControl component is a particularly long string then if overlays the down arrow icon.

This PR adds sufficient padding to the RHS of the control to ensure the icon has room to display (see fix below).

How has this been tested?

• Added Storybook story.
• Visually verified issue fixed.

Testing Instructions

On Master

• Go to latest Storybook for CustomSelectControl..
• Select an option from the control dropdown.
• Using devtools amend the select option text to a really long string (see screencapture below for instructions on how to do this).
• See dropdown arrow is overlayed by the text.

On this PR

• Checkout PR and build Gutenberg npm run dev.
• Run local Storybook - npm run storybook:dev.
• Navigate to CustomSelectControl stories in Storybook. Chose the "Long labels" story.
• See that despite using really long option labels the arrow is not overlayed by the option text.

Types of changes

Bug fix (non-breaking change which fixes an issue).

Checklist:

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

[github-actions]

Size Change: +383 B (0%)

Total Size: 1.12 MB

Filename	Size	Change
build/block-editor/index.js	106 kB	+272 B (0%)
build/block-library/index.js	126 kB	+32 B (0%)
build/block-library/style-rtl.css	7.72 kB	+31 B (0%)
build/block-library/style.css	7.72 kB	+31 B (0%)
build/components/style-rtl.css	19.5 kB	+9 B (0%)
build/components/style.css	19.5 kB	+8 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
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build/annotations/index.js	3.62 kB	0 B
build/api-fetch/index.js	3.4 kB	0 B
build/autop/index.js	2.83 kB	0 B
build/blob/index.js	620 B	0 B
build/block-directory/index.js	6.75 kB	0 B
build/block-directory/style-rtl.css	892 B	0 B
build/block-directory/style.css	892 B	0 B
build/block-editor/style-rtl.css	11.4 kB	0 B
build/block-editor/style.css	11.4 kB	0 B
build/block-library/editor-rtl.css	7.87 kB	0 B
build/block-library/editor.css	7.88 kB	0 B
build/block-library/theme-rtl.css	684 B	0 B
build/block-library/theme.css	686 B	0 B
build/block-serialization-default-parser/index.js	1.88 kB	0 B
build/block-serialization-spec-parser/index.js	3.1 kB	0 B
build/blocks/index.js	48.2 kB	0 B
build/components/index.js	193 kB	0 B
build/compose/index.js	9.31 kB	0 B
build/core-data/index.js	11.4 kB	0 B
build/data-controls/index.js	1.29 kB	0 B
build/data/index.js	8.46 kB	0 B
build/date/index.js	5.47 kB	0 B
build/deprecated/index.js	771 B	0 B
build/dom-ready/index.js	568 B	0 B
build/dom/index.js	3.17 kB	0 B
build/edit-navigation/index.js	8.25 kB	0 B
build/edit-navigation/style-rtl.css	918 B	0 B
build/edit-navigation/style.css	919 B	0 B
build/edit-post/index.js	302 kB	0 B
build/edit-post/style-rtl.css	5.43 kB	0 B
build/edit-post/style.css	5.43 kB	0 B
build/edit-site/index.js	15 kB	0 B
build/edit-site/style-rtl.css	2.96 kB	0 B
build/edit-site/style.css	2.96 kB	0 B
build/edit-widgets/index.js	8.83 kB	0 B
build/edit-widgets/style-rtl.css	2.4 kB	0 B
build/edit-widgets/style.css	2.4 kB	0 B
build/editor/editor-styles-rtl.css	425 B	0 B
build/editor/editor-styles.css	428 B	0 B
build/editor/index.js	44.7 kB	0 B
build/editor/style-rtl.css	4.26 kB	0 B
build/editor/style.css	4.27 kB	0 B
build/element/index.js	4.65 kB	0 B
build/escape-html/index.js	733 B	0 B
build/format-library/index.js	7.72 kB	0 B
build/format-library/style-rtl.css	502 B	0 B
build/format-library/style.css	502 B	0 B
build/hooks/index.js	2.13 kB	0 B
build/html-entities/index.js	621 B	0 B
build/i18n/index.js	3.56 kB	0 B
build/is-shallow-equal/index.js	711 B	0 B
build/keyboard-shortcuts/index.js	2.52 kB	0 B
build/keycodes/index.js	1.94 kB	0 B
build/list-reusable-blocks/index.js	3.12 kB	0 B
build/list-reusable-blocks/style-rtl.css	226 B	0 B
build/list-reusable-blocks/style.css	226 B	0 B
build/media-utils/index.js	5.3 kB	0 B
build/notices/index.js	1.79 kB	0 B
build/nux/index.js	3.4 kB	0 B
build/nux/style-rtl.css	616 B	0 B
build/nux/style.css	613 B	0 B
build/plugins/index.js	2.56 kB	0 B
build/primitives/index.js	1.5 kB	0 B
build/priority-queue/index.js	789 B	0 B
build/redux-routine/index.js	2.85 kB	0 B
build/rich-text/index.js	14.8 kB	0 B
build/server-side-render/index.js	2.68 kB	0 B
build/shortcode/index.js	1.7 kB	0 B
build/token-list/index.js	1.28 kB	0 B
build/url/index.js	4.06 kB	0 B
build/viewport/index.js	1.85 kB	0 B
build/warning/index.js	1.14 kB	0 B
build/wordcount/index.js	1.17 kB	0 B

_{compressed-size-action}

[talldan]

Looks good, thanks for working this into a separate PR.

Let's update to use the $icon-size variable then merge.

[marekhrabe]

Notes seem addressed now