Font library: load font assets on editor canvas

This change adds loaded font faces to the editor canvas iframe so that they can be used immediately after uploading them.

Re: #51764

packages/edit-site/src/components/global-styles/font-library-modal/local-fonts.js

@@ -114,13 +114,18 @@ function LocalFonts() {
 	};
 
 	const addFontFaceToBrowser = async ( fontFaceData ) => {
+		const editorCanvas = document.querySelector(
+			'iframe[name="editor-canvas"]'
+		);
+		const iframeDocument = editorCanvas.contentDocument;
 		const data = await fontFaceData.file.arrayBuffer();
 		const newFont = new window.FontFace( fontFaceData.fontFamily, data, {
 			style: fontFaceData.fontStyle,
 			weight: fontFaceData.fontWeight,
 		} );
 		const loadedFace = await newFont.load();
 		document.fonts.add( loadedFace );
+		iframeDocument.fonts.add( loadedFace );
 	};
 
 	/**