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/assistant/src/context.rs

@@ -1123,16 +1123,17 @@ impl Context {
                     .timer(Duration::from_millis(200))
                     .await;
 
-                let token_count = cx
-                    .update(|cx| {
-                        LanguageModelCompletionProvider::read_global(cx).count_tokens(request, cx)
-                    })?
-                    .await?;
+                if let Some(token_count) = cx.update(|cx| {
+                    LanguageModelCompletionProvider::read_global(cx).count_tokens(request, cx)
+                })? {
+                    let token_count = token_count.await?;
+
+                    this.update(&mut cx, |this, cx| {
+                        this.token_count = Some(token_count);
+                        cx.notify()
+                    })?;
+                }
 
-                this.update(&mut cx, |this, cx| {
-                    this.token_count = Some(token_count);
-                    cx.notify()
-                })?;
                 anyhow::Ok(())
             }
             .log_err()