Add missing property to Pickadate (#23217)

* Add missing property to Pickadate

Add `weekdaysLetter` property to Pickadate.DateOptions
https://github.com/Dogfalo/materialize/blob/6da429c5972c90e386e34f14bed3f17c8c40fa95/dist/js/materialize.js#L5905

* Update index.d.ts

types/materialize-css/v0/index.d.ts

@@ -5,6 +5,7 @@
 //                  Sukhdeep Singh <https://github.com/SinghSukhdeep>
 //                  Jean-Francois Cere <https://github.com/jfcere>
 //                  Sebastien Cote <https://github.com/scote>
+//                  Liad Idan <https://github.com/LiadIdan>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 // TypeScript Version: 2.3
 
@@ -475,11 +476,15 @@ declare namespace Materialize {
 }
 
 /**
- * Declare Pickadate namespace again in order to add more Materialize specific properties to TimeOptions interface
+ * Declare Pickadate namespace again in order to add more Materialize specific properties to TimeOptions and DateOptions interfaces
  *
  * @see http://www.typescriptlang.org/docs/handbook/declaration-merging.html
  */
 declare namespace Pickadate {
+	interface DateOptions {
+		weekdaysLetter?: string[];
+	}
+
 	interface TimeOptions {
 		/**
 		 * Set default time such as : 'now', '1:30AM', '16:30'.