editor: Improve snippet completion to show key inline in completion and description as aside (#30603)

Closes #28028

Before:
<img width="742" alt="image"
src="https://github.com/user-attachments/assets/31723970-5420-40ea-a394-4ffa0038925c"
/>

After:
<img width="989" alt="image"
src="https://github.com/user-attachments/assets/0aebc317-a234-4e68-8304-cb479513af15"
/>


Release Notes:

- Improved snippet code completion to show key in completion menu and
description in aside.

crates/project/src/lsp_store.rs

@@ -9897,6 +9897,11 @@ pub enum CompletionDocumentation {
     MultiLinePlainText(SharedString),
     /// Markdown documentation.
     MultiLineMarkdown(SharedString),
+    /// Both single line and multiple lines of plain text documentation.
+    SingleLineAndMultiLinePlainText {
+        single_line: SharedString,
+        plain_text: Option<SharedString>,
+    },
 }
 
 impl From<lsp::Documentation> for CompletionDocumentation {