Add support for insert_text_mode of a completion (#28171)

I wanted this for CONL (https://conl.dev )'s nascent langauge server,
and it seems like most of the support was already wired up on the LSP
side, so this surfaces it into the editor.

Release Notes:

- Added support for the `insert_text_mode` field of completions from the
language server protocol.

crates/collab_ui/src/chat_panel/message_editor.rs

@@ -315,6 +315,7 @@ impl MessageEditor {
                     icon_path: None,
                     confirm: None,
                     documentation: None,
+                    insert_text_mode: None,
                     source: CompletionSource::Custom,
                 }
             })