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/shared_string.rs

@@ -1,4 +1,5 @@
 use derive_more::{Deref, DerefMut};
+
 use serde::{Deserialize, Serialize};
 use std::{borrow::Borrow, sync::Arc};
 use util::arc_cow::ArcCow;