[MM-33112] Add support for multiple custom spellchecking languages (#1743)

* [MM-33112] Add support for multiple custom spellchecking languages

* Styles and other formatting

* Type and lint fixes

* Update wording

src/common/communication.ts

@@ -98,3 +98,5 @@ export const SEND_DROPDOWN_MENU_SIZE = 'send-dropdown-menu-size';
 
 export const BROWSER_HISTORY_PUSH = 'browser-history-push';
 export const APP_LOGGED_IN = 'app-logged-in';
+
+export const GET_AVAILABLE_SPELL_CHECKER_LANGUAGES = 'get-available-spell-checker-languages';