Icons: Improve the SVG sanitizer in WP_Icons_Registry

[wildworks]

• Follow-up to #64847.
• Gutenberg PR: ​https://github.com/WordPress/gutenberg/pull/75550

Core still has no dedicated SVG sanitization function, so the icons registry sanitizes icon content on its own. That sanitizer is incomplete: it strips attributes and elements that are valid SVG markup.

This ticket improves the sanitizer by extracting the root SVG element with WP_HTML_Processor before wp_kses() runs, and by expanding the allow-list to cover SVG-friendly elements and attributes.

• Trac ticket: https://core.trac.wordpress.org/ticket/65795
• Gutenberg PR: ​https://github.com/WordPress/gutenberg/pull/75550

## Use of AI Tools

Used for: Initial code skeleton and test suggestions; final implementation and tests were reviewed and edited by me.

@t-hamano with ​https://github.com/WordPress/gutenberg/pull/78808 merged, can you take care of the Core side of things? Ie.

• rebase onto latest trunk
• Add ​https://github.com/WordPress/gutenberg/pull/78808 to its description (together with ​https://github.com/WordPress/gutenberg/pull/75550)
• Confirm its tests cover the stroke, style, vector-effect, and clip-rule attributes.
• Anything else to bring it up to date

Thank you 🙏