WebComponentExporter should copy @font-face declarations to document body in V25

[rolfsmeds]

Describe your motivation

In V24, a separate document.css could be used in a theme loaded into an exported component to load certain css into the document body instead of the exporter component's shadow DOM, as some css doesn't work inside shadow DOM.

In 2025, the only css that still needs to be in the document root is @font-face declarations. In V25, the entire theme folder concept is retired, including document.css, so a different solution is needed for loading fonts as part of exported components.

Describe the solution you'd like

Any @font-face declarations found in stylesheets loaded into exported components (using @CssImport, as @Theme is deprecated) should be automatically copied (moved?) to the document root.

Related: #21272