Allow Vector2 to be instantiated with one argument

lib/vector2.js

@@ -3,11 +3,14 @@ import { jitter, random } from './random.js'
 export class Vector2 {
   /**
    * @param {number} x coordinate
-   * @param {number} y coordinate
+   * @param {number} [y] coordinate defaults to `x` if omitted.
    */
   constructor(x, y) {
+    if (typeof x !== 'number') {
+      throw new Error(`Vector2 constructor requires a number for x, got ${typeof x}`)
+    }
     this.x = x
-    this.y = y
+    this.y = y ?? x
   }
 
   /**
@@ -154,9 +157,9 @@ export class Vector2 {
 
 /**
  * @param {number} x 
- * @param {number} y 
+ * @param {number} [y] defaults to `x` if omitted. 
  * @returns Vector2
  */
 export function vec2(x, y) {
-  return new Vector2(x, y)
+  return new Vector2(x, y ?? x)
 }