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/assistant_context_editor/src/slash_command.rs

@@ -127,6 +127,7 @@ impl SlashCommandCompletionProvider {
                                 new_text,
                                 label: command.label(cx),
                                 icon_path: None,
+                                insert_text_mode: None,
                                 confirm,
                                 source: CompletionSource::Custom,
                             })
@@ -228,6 +229,7 @@ impl SlashCommandCompletionProvider {
                                 new_text,
                                 documentation: None,
                                 confirm,
+                                insert_text_mode: None,
                                 source: CompletionSource::Custom,
                             }
                         })