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/completion/src/completion.rs

@@ -1,5 +1,5 @@
 use anyhow::{anyhow, Result};
-use futures::{future::BoxFuture, stream::BoxStream, FutureExt, StreamExt};
+use futures::{future::BoxFuture, stream::BoxStream, StreamExt};
 use gpui::{AppContext, Global, Model, ModelContext, Task};
 use language_model::{
     LanguageModel, LanguageModelProvider, LanguageModelProviderId, LanguageModelRegistry,
@@ -143,11 +143,11 @@ impl LanguageModelCompletionProvider {
         &self,
         request: LanguageModelRequest,
         cx: &AppContext,
-    ) -> BoxFuture<'static, Result<usize>> {
+    ) -> Option<BoxFuture<'static, Result<usize>>> {
         if let Some(model) = self.active_model() {
-            model.count_tokens(request, cx)
+            Some(model.count_tokens(request, cx))
         } else {
-            std::future::ready(Err(anyhow!("No active model set"))).boxed()
+            None
         }
     }