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/gpui/src/text_system.rs

@@ -1,8 +1,10 @@
+mod font_fallbacks;
 mod font_features;
 mod line;
 mod line_layout;
 mod line_wrapper;
 
+pub use font_fallbacks::*;
 pub use font_features::*;
 pub use line::*;
 pub use line_layout::*;
@@ -62,8 +64,7 @@ impl TextSystem {
             wrapper_pool: Mutex::default(),
             font_runs_pool: Mutex::default(),
             fallback_font_stack: smallvec![
-                // TODO: This is currently Zed-specific.
-                // We should allow GPUI users to provide their own fallback font stack.
+                // TODO: Remove this when Linux have implemented setting fallbacks.
                 font("Zed Plex Mono"),
                 font("Helvetica"),
                 font("Segoe UI"),  // Windows
@@ -683,6 +684,9 @@ pub struct Font {
     /// The font features to use.
     pub features: FontFeatures,
 
+    /// The fallbacks fonts to use.
+    pub fallbacks: Option<FontFallbacks>,
+
     /// The font weight.
     pub weight: FontWeight,
 
@@ -697,6 +701,7 @@ pub fn font(family: impl Into<SharedString>) -> Font {
         features: FontFeatures::default(),
         weight: FontWeight::default(),
         style: FontStyle::default(),
+        fallbacks: None,
     }
 }