Overhaul handling of font families

* Specify font families in the theme.
* Load fonts eagerly when loading themes, instead of loading
  them lazily when rendering.

Co-Authored-By: Antonio Scandurra <me@as-cii.com>
Co-Authored-By: Nathan Sobo <nathan@zed.dev>

zed/src/editor/display_map/wrap_map.rs

@@ -921,7 +921,7 @@ mod tests {
             tab_size: rng.gen_range(1..=4),
             buffer_font_family: font_cache.load_family(&["Helvetica"]).unwrap(),
             buffer_font_size: 14.0,
-            ..Settings::new(&font_cache).unwrap()
+            ..cx.read(Settings::test)
         };
         log::info!("Tab size: {}", settings.tab_size);
         log::info!("Wrap width: {:?}", wrap_width);