Refactor Completions to allow non-LSP ones better (#26300)

A preparation for https://github.com/zed-industries/zed/issues/4957 that
pushes all LSP-related data out from the basic completion item, so that
it's possible to create completion items without any trace of LSP
clearly.

Release Notes:

- N/A

crates/proto/proto/zed.proto

@@ -1,6 +1,6 @@
-
 syntax = "proto3";
 package zed.messages;
+import "google/protobuf/wrappers.proto";
 
 // Looking for a number? Search "// current max"
 
@@ -999,6 +999,12 @@ message Completion {
     uint64 server_id = 4;
     bytes lsp_completion = 5;
     bool resolved = 6;
+    Source source = 7;
+
+    enum Source {
+        Custom = 0;
+        Lsp = 1;
+    }
 }
 
 message GetCodeActions {