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/styled.rs

@@ -506,6 +506,7 @@ pub trait Styled: Sized {
         let Font {
             family,
             features,
+            fallbacks,
             weight,
             style,
         } = font;
@@ -515,6 +516,7 @@ pub trait Styled: Sized {
         text_style.font_features = Some(features);
         text_style.font_weight = Some(weight);
         text_style.font_style = Some(style);
+        text_style.font_fallbacks = fallbacks;
 
         self
     }