Fix date format to 'en-US' (#1741)

extensions/ql-vscode/src/pure/date.ts

@@ -2,14 +2,14 @@
  * Contains an assortment of helper constants and functions for working with dates.
  */
 
-const dateWithoutYearFormatter = new Intl.DateTimeFormat(undefined, {
+const dateWithoutYearFormatter = new Intl.DateTimeFormat('en-US', {
   month: 'short',
   day: 'numeric',
   hour: 'numeric',
   minute: '2-digit',
 });
 
-const dateFormatter = new Intl.DateTimeFormat(undefined, {
+const dateFormatter = new Intl.DateTimeFormat('en-US', {
   year: 'numeric',
   month: 'short',
   day: 'numeric',