Allow Edge arrows from to be left out of edge options

network.setOptions({edges: {arrows: {to: true}}}); should be accepted since it works properly in vis, but previous @types/vis will fail type-checking.

types/vis/index.d.ts

@@ -1861,7 +1861,7 @@ export interface EdgeOptions {
       enabled?: boolean,
       scaleFactor?: number,
     },
-    from: boolean | {
+    from?: boolean | {
       enabled?: boolean,
       scaleFactor?: number,
     }