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_bindings/codegen/Bindings.conf

@@ -175,6 +175,10 @@ DOMInterfaces = {
     'canGc': ['Abort'],
 },
 
+'FontFaceSet': {
+    'canGc': ['Load'],
+},
+
 'GamepadHapticActuator': {
     'inRealms': ['PlayEffect', 'Reset'],
     'canGc': ['PlayEffect', 'Reset'],
@@ -577,6 +581,10 @@ DOMInterfaces = {
 }
 
 Dictionaries = {
+'FontFaceDescriptors': {
+    'derives': ['Clone', 'MallocSizeOf']
+},
+
 'GPUCanvasConfiguration': {
     'derives': ['Clone']
 },