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/language/src/proto.rs

@@ -79,11 +79,13 @@ pub fn serialize_operation(operation: &crate::Operation) -> proto::Operation {
             crate::Operation::UpdateCompletionTriggers {
                 triggers,
                 lamport_timestamp,
+                server_id,
             } => proto::operation::Variant::UpdateCompletionTriggers(
                 proto::operation::UpdateCompletionTriggers {
                     replica_id: lamport_timestamp.replica_id as u32,
                     lamport_timestamp: lamport_timestamp.value,
-                    triggers: triggers.clone(),
+                    triggers: triggers.iter().cloned().collect(),
+                    language_server_id: server_id.to_proto(),
                 },
             ),
         }),
@@ -326,6 +328,7 @@ pub fn deserialize_operation(message: proto::Operation) -> Result<crate::Operati
                         replica_id: message.replica_id as ReplicaId,
                         value: message.lamport_timestamp,
                     },
+                    server_id: LanguageServerId::from_proto(message.language_server_id),
                 }
             }
         },