make duration optional in types

svg.js.d.ts

 >
 
 // create our own style declaration that includes css vars
-export interface CSSStyleDeclarationWithVars extends CSSStyleDeclaration {
+interface CSSStyleDeclarationWithVars extends CSSStyleDeclaration {
   [key: `--${string}`]: string
 }
 
     element(el: Element): this
     timeline(): Timeline
     timeline(timeline: Timeline): this
-    animate(duration: TimeLike, delay?: number, when?: string): this
+    animate(duration?: TimeLike, delay?: number, when?: string): this
     schedule(delay: number, when?: string): this
     schedule(timeline: Timeline, delay?: number, when?: string): this
     unschedule(): this