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/language/src/outline.rs

@@ -162,6 +162,7 @@ pub fn render_item<T>(
         color: cx.theme().colors().text,
         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(1.),