Icons: keep fill and stroke on the sanitized <svg> root

[retrofox]

What

sanitize_icon_content() strips fill and stroke from the root <svg>, so registered icon content loses fill="currentColor" and stops inheriting the surrounding color. Every icon in packages/icons declares it on the root.

This allows both attributes: an override in WP_Icons_Registry_Gutenberg, which is what runs with the plugin active, and the same change in the 7.0 compat copy. Core's copy needs the same two attributes when the iteration syncs.

Gap discussed in #75715 (comment), surfaced with screenshots in this review thread.

Why

Sanitized icons fall back to the default black fill: near-invisible against dark schemes, unresponsive to currentColor, and every consumer has to re-add the attribute client-side.

How

• WP_Icons_Registry_Gutenberg
  Overrides sanitize_icon_content() with fill and stroke allowed on the svg root.

• Compat
  Mirrors the allow-list change in lib/compat/wordpress-7.0/class-wp-icons-registry.php.

• Tests
  test_register_icon_keeps_root_color_attributes asserts both attributes survive registration.

Testing

1. With the plugin active, check a core icon keeps its fill:

wp eval 'echo wp_get_icon( "core/calendar" );'

The root <svg> carries fill="currentColor".

2. GET /wp/v2/icons/core/calendar: content carries the attribute as well.

3. Run the registry tests:

phpunit --filter Icons_Registry_Gutenberg

Follow-ups

• Apply the same allow-list change to Core's WP_Icons_Registry with the 7.1 sync (Core-64847).

[retrofox]

closing in favor of #75550

[t-hamano]

Since #75550 might not make it in time for 7.1, I think we can proceed with this PR first. Can you also prepare the core PR along with this one? The core ticket is here: https://core.trac.wordpress.org/ticket/64847