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/editor.rs

@@ -12430,6 +12430,7 @@ impl Render for Editor {
                 color: cx.theme().colors().editor_foreground,
                 font_family: settings.ui_font.family.clone(),
                 font_features: settings.ui_font.features.clone(),
+                font_fallbacks: settings.ui_font.fallbacks.clone(),
                 font_size: rems(0.875).into(),
                 font_weight: settings.ui_font.weight,
                 line_height: relative(settings.buffer_line_height.value()),
@@ -12439,6 +12440,7 @@ impl Render for Editor {
                 color: cx.theme().colors().editor_foreground,
                 font_family: settings.buffer_font.family.clone(),
                 font_features: settings.buffer_font.features.clone(),
+                font_fallbacks: settings.buffer_font.fallbacks.clone(),
                 font_size: settings.buffer_font_size(cx).into(),
                 font_weight: settings.buffer_font.weight,
                 line_height: relative(settings.buffer_line_height.value()),

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(),
     };