[jquery.tooltipster] Added definition for 'side' option

Definition description is taken from the project page and tested with
Typescript 4.1.

types/jquery.tooltipster/index.d.ts

@@ -1,6 +1,9 @@
 // Type definitions for jQuery Tooltipster 3.3.0
 // Project: https://github.com/iamceege/tooltipster
-// Definitions by: Patrick Magee <https://github.com/pjmagee/>, Dmitry Pesterev <https://github.com/VorobeY1326/>, Leonard Thieu <https://github.com/leonard-thieu>
+// Definitions by: Patrick Magee <https://github.com/pjmagee/>,
+//                 Dmitry Pesterev <https://github.com/VorobeY1326/>,
+//                 Leonard Thieu <https://github.com/leonard-thieu>,
+//                 Jan Hirzel <https://github.com/janhi/>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 // TypeScript Version: 2.3
 
@@ -137,6 +140,10 @@ interface JQueryTooltipsterOptions {
     */
     restoration?: string;
     /**
+    * Sets the side of the tooltip. The value may one of the following: 'top', 'bottom', 'left', 'right'. It may also be an array containing one or more of these values. When using an array, the order of values is taken into account as order of fallbacks and the absence of a side disables it (see the sides section). Default: ['top', 'bottom', 'right', 'left']
+    */
+    side?: string | string[];
+    /**
     * Set the speed of the animation. Default: 350
     */
     speed?: number;