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/editor/src/test.rs

@@ -27,6 +27,7 @@ pub fn marked_display_snapshot(
     let font = Font {
         family: "Zed Plex Mono".into(),
         features: FontFeatures::default(),
+        fallbacks: None,
         weight: FontWeight::default(),
         style: FontStyle::default(),
     };