lsp: Track completion triggers for each language separately (#20471)

This PR improves how we handle completions in buffers with multiple
LSPs.

Context: while working on
https://github.com/zed-industries/zed/issues/19777 with @mgsloan we
noticed that completion triggers coming from language servers are not
tracked properly. Namely, each buffer has `completion_triggers` field
which is read from the configuration of a language server. The problem
is, there can be multiple language servers for a single buffer, in which
case we'd just stick to the one that was registered last.

This PR makes the tracking a bit more fine-grained. We now track not
only what the completion triggers are, but also their origin server id.
Whenever completion triggers are updated, we recreate the completion
triggers set.
Release Notes:

- Fixed completions not triggering when multiple language servers are
used for a single file.

crates/editor/src/editor.rs

@@ -13700,10 +13700,7 @@ impl CompletionProvider for Model<Project> {
             return true;
         }
 
-        buffer
-            .completion_triggers()
-            .iter()
-            .any(|string| string == text)
+        buffer.completion_triggers().contains(text)
     }
 }