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/font_fallbacks.rs

@@ -0,0 +1,21 @@
+use std::sync::Arc;
+
+use schemars::JsonSchema;
+use serde_derive::{Deserialize, Serialize};
+
+/// The fallback fonts that can be configured for a given font.
+/// Fallback fonts family names are stored here.
+#[derive(Default, Clone, Eq, PartialEq, Hash, Debug, Deserialize, Serialize, JsonSchema)]
+pub struct FontFallbacks(pub Arc<Vec<String>>);
+
+impl FontFallbacks {
+    /// Get the fallback fonts family names
+    pub fn fallback_list(&self) -> &[String] {
+        &self.0.as_slice()
+    }
+
+    /// Create a font fallback from a list of strings
+    pub fn from_fonts(fonts: Vec<String>) -> Self {
+        FontFallbacks(Arc::new(fonts))
+    }
+}