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/lsp/src/lsp.rs

@@ -704,8 +704,15 @@ impl LanguageServer {
                             }),
                             insert_replace_support: Some(true),
                             label_details_support: Some(true),
+                            insert_text_mode_support: Some(InsertTextModeSupport {
+                                value_set: vec![
+                                    InsertTextMode::AS_IS,
+                                    InsertTextMode::ADJUST_INDENTATION,
+                                ],
+                            }),
                             ..Default::default()
                         }),
+                        insert_text_mode: Some(InsertTextMode::ADJUST_INDENTATION),
                         completion_list: Some(CompletionListCapability {
                             item_defaults: Some(vec![
                                 "commitCharacters".to_owned(),