[API] Add reroute convenience methods (#812)

Adds `LGraph.getReroute`, which accepts null & undefined IDs. Interface
overloads result in `never` when given a _known_ `undefined` or `null`
value.

src/LGraph.ts

@@ -1398,6 +1398,17 @@ export class LGraph implements LinkNetwork, Serialisable<SerialisableGraph> {
     }
   }
 
+  /**
+   * Returns a reroute by ID, or `undefined` if the ID is `null` or `undefined`.
+   * @param id ID of reroute to return
+   * @returns The reroute with the provided {@link id}, otherwise `undefined`. Always returns `undefined` if `id` is `null` or `undefined`.
+   */
+  getReroute(id: null | undefined): undefined
+  getReroute(id: RerouteId | null | undefined): Reroute | undefined
+  getReroute(id: RerouteId | null | undefined): Reroute | undefined {
+    return id == null ? undefined : this.reroutes.get(id)
+  }
+
   /**
    * Configures a reroute on the graph where ID is already known (probably deserialisation).
    * Creates the object if it does not exist.