Move Day.js plugin types import to global typings (#2954)

framework/core/js/@types/global/index.d.ts

@@ -3,6 +3,7 @@ import Mithril from 'mithril';
 
 // Other third-party libs
 import * as _dayjs from 'dayjs';
+import 'dayjs/plugin/relativeTime';
 import * as _$ from 'jquery';
 
 // Globals from flarum/core

framework/core/js/src/common/utils/humanTime.ts

@@ -1,5 +1,4 @@
 import dayjs from 'dayjs';
-import 'dayjs/plugin/relativeTime';
 
 /**
  * The `humanTime` utility converts a date to a localized, human-readable time-