Update preview key location to match new schema

[mikachan]

This PR updates the preview key location in line with recent changes to the font collection schema in WordPress/gutenberg#58413.

The schema has been changed to the following structure:

		{
			"preview": "https://s.w.org/images/fonts/17.6/previews/abeezee/abeezee.svg",
			"font_family_settings": {
				"name": "ABeeZee",
				"fontFamily": "ABeeZee, sans-serif",
				"slug": "abeezee"
			},
			"categories": [ "sans-serif" ],
			"font_faces": [
				{
					"preview": "https://s.w.org/images/fonts/17.6/previews/abeezee/abeezee-400-normal.svg",
					"font_face_settings": {
						"src": "https://fonts.gstatic.com/s/abeezee/v22/esDR31xSG-6AGleN6teukbcHCpE.woff2",
						"fontWeight": "400",
						"fontStyle": "normal",
						"fontFamily": "ABeeZee"
					}
				}
			]
		}

Note that the preview for the font family now sits on the top level, and the preview for each font face sits next to a new key which describes each font face, font_face_settings.

I also ran the files and previews script again, which included a few updates from Google Fonts.

[matiasbenedetto]

✔️ The JSON files match the new schema.
The change looks good to me.