Add padding control for custom SVG icons

[selul]

Summary

• Add an SVG Padding field whose untouched default is unset and displayed with a 0 placeholder.
• Emit padding and box-sizing: border-box for any explicitly selected numeric value, including 0.
• Leave existing icons with missing metadata, and new icons whose padding is untouched, with no padding-related CSS or default rendering changes.
• Apply configured padding consistently in editor previews and frontend output.
• Validate numeric icon style metadata and allow-list vertical alignment values before generating inline CSS.
• Add regression coverage for positive, explicit zero, unset, missing, and unsafe stored values.

Will affect visual aspect of the product

YES, only after a user explicitly configures SVG padding.

Test instructions

• Confirm an existing custom SVG menu icon has no padding-related inline CSS and renders unchanged after updating.
• Create a new custom SVG icon and leave SVG Padding untouched; confirm it has no padding-related inline CSS.
• Explicitly enter 0; confirm the preview and frontend emit padding:0px plus box-sizing:border-box.
• Enter a positive value; confirm the preview and frontend emit that padding plus box-sizing:border-box without changing the configured outer width.
• Store a non-numeric value for padding, width, or font size and confirm it is omitted from the generated inline style.

Verification

• Focused PHPUnit: 3 tests, 11 assertions passed.
• GitHub Actions PHP Lint and PHPUnit jobs passed.
• PHP CodeSniffer passed under PHP 8.0; PHP syntax checks and git diff --check also passed.

Check before Pull Request is ready:

• Regression tests are included in this PR.
• The PHPUnit and PHP lint CI jobs pass.
• PHP CodeSniffer passes under PHP 8.0.
• The changed PHP follows the surrounding WordPress coding style.
• Security and sanitization requirements have been reviewed.
• A reviewer still needs to be assigned.

Closes #344.

[pirate-bot]

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