script: add skeleton implementation of FontFace API (#35262)

This patch implements the `FontFace` interface, but with some caveats

1. The interface is only exposed on `Window`. Support for Workers will
   be handled in the future.
2. The concept of `css-connected` `FontFace` is not implemented, so
   `@font-face` rules in stylesheets will not be represented in the DOM.
3. The constructor only supports using `url()` strings as source
   and `ArrayBuffer` and `ArrayBufferView` are not supported yet.

A skeleton implementation of the `load` method of `FontFaceSet` is also
implemented in this patch. The intention is to support some web pages
that don't load without this method.

Signed-off-by: Mukilan Thiyagarajan <mukilan@igalia.com>

components/script/dom/window.rs

@@ -634,6 +634,10 @@ impl Window {
     ) -> EventStatus {
         event.dispatch(self.upcast(), true, can_gc)
     }
+
+    pub(crate) fn font_context(&self) -> &Arc<FontContext> {
+        &self.font_context
+    }
 }
 
 // https://html.spec.whatwg.org/multipage/#atob