Feature/fallback fonts (#15306)

Supersedes https://github.com/zed-industries/zed/pull/12090

fixes #5180
fixes #5055

See original PR for an example of the feature at work.

This PR changes the settings interface to be backwards compatible, and
adds the `ui_font_fallbacks`, `buffer_font_fallbacks`, and
`terminal.font_fallbacks` settings.

Release Notes:

- Added support for font fallbacks via three new settings:
`ui_font_fallbacks`, `buffer_font_fallbacks`, and
`terminal.font_fallbacks`.(#5180, #5055).

---------

Co-authored-by: Junkui Zhang <364772080@qq.com>

crates/settings/src/settings_file.rs

@@ -18,9 +18,11 @@ pub fn test_settings() -> String {
             "ui_font_family": "Courier",
             "ui_font_features": {},
             "ui_font_size": 14,
+            "ui_font_fallback": [],
             "buffer_font_family": "Courier",
             "buffer_font_features": {},
             "buffer_font_size": 14,
+            "buffer_font_fallback": [],
             "theme": EMPTY_THEME_NAME,
         }),
         &mut value,