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/editor/src/code_context_menus.rs

@@ -240,6 +240,7 @@ impl CompletionsMenu {
                 icon_path: None,
                 documentation: None,
                 confirm: None,
+                insert_text_mode: None,
                 source: CompletionSource::Custom,
             })
             .collect();