Improve SVG Icon block for 1.4.8

[wpgaurav]

What changed

• register the SVG Icon block from block.json while preserving the existing block name and shortcode
• add bounded px, em, and rem sizing, native color/spacing/anchor/class supports, original-color and monochrome modes, and decorative/informative accessibility controls
• replace eager editor localization with a paginated REST picker, debounced search, recent-icon ordering, keyboard semantics, and missing-icon recovery
• harden SVG sanitization, restrict local references and style properties, sanitize REST output, and prefix definition IDs per rendered instance
• add focused sanitizer/rendering regression tests and update 1.4.8 documentation

Why

The previous block eagerly loaded the full icon library, hand-maintained block metadata, forced all path fills, offered only pixel sizing, and always hid icons from assistive technology. Its regex-based renderer could also duplicate root attributes and SVG definition IDs.

Validation

• composer check (PHP lint, WordPress Coding Standards, 24 PHPUnit tests / 94 assertions)
• JavaScript syntax validation with node --check
• 1.4.8 distribution build and ZIP integrity check
• WordPress Studio MCP on WordPress 7.0.2 / PHP 8.4
  • plugin installed from the built ZIP
  • block metadata and REST route registered
  • anonymous REST denied; editor REST request returned sanitized results
  • real editor validation: 5/5 and 2/2 blocks valid with no serialization changes
  • frontend verified at desktop and mobile sizes
  • decorative/informative ARIA, original/monochrome paint, px/em/rem sizing, anchor, custom class, spacing, and shortcode compatibility all passed