workspace: Add trailing / to directories on completion when using OpenPathPrompt (#25430)

Closes #25045

With the setting `"use_system_path_prompts": false`, previously, if the
completion target was a directory, no separator would be added after it,
requiring us to manually append a `/` or `\`. Now, if the completion
target is a directory, a `/` or `\` will be automatically added. On
Windows, both `/` and `\` are considered valid path separators.



https://github.com/user-attachments/assets/0594ce27-9693-4a49-ae0e-3ed29f62526a



Release Notes:

- N/A

crates/proto/proto/zed.proto

@@ -572,13 +572,23 @@ message JoinProject {
     uint64 project_id = 1;
 }
 
+message ListRemoteDirectoryConfig {
+    bool is_dir = 1;
+}
+
 message ListRemoteDirectory {
     uint64 dev_server_id = 1;
     string path = 2;
+    ListRemoteDirectoryConfig config = 3;
+}
+
+message EntryInfo {
+    bool is_dir = 1;
 }
 
 message ListRemoteDirectoryResponse {
     repeated string entries = 1;
+    repeated EntryInfo entry_info = 2;
 }
 
 message JoinProjectResponse {