Improve importing font-family settings from VS Code (#39736)

Closes https://github.com/zed-industries/zed/issues/39259

- Fixes import of `editor.fontFamily` (we were looking for the wrong
key)
- Adds basic support for the CSS font-family syntax used by VS Code,
including font fallback

Release Notes:

- N/A

crates/settings/src/settings_file.rs

@@ -22,7 +22,7 @@ pub fn test_settings() -> String {
             "buffer_font_family": "Courier",
             "buffer_font_features": {},
             "buffer_font_size": 14,
-            "buffer_font_fallback": [],
+            "buffer_font_fallbacks": [],
             "theme": EMPTY_THEME_NAME,
         }),
         &mut value,
@@ -37,7 +37,7 @@ pub fn test_settings() -> String {
             "buffer_font_family": "Courier New",
             "buffer_font_features": {},
             "buffer_font_size": 14,
-            "buffer_font_fallback": [],
+            "buffer_font_fallbacks": [],
             "theme": EMPTY_THEME_NAME,
         }),
         &mut value,