Add declaration file for ColorThief

Include declaration for ColorThief since it is written in JavaScript
only.

packages/web-kitchensink-reactnext/colorthief.d.ts

@@ -0,0 +1,5 @@
+declare module 'colorthief' {
+  export default class ColorThief {
+    getPalette(image: HTMLImageElement, colorCount: number): Promise<[number, number, number][]>;
+  }
+}

packages/web-kitchensink-reactnext/src/utils/image.ts

@@ -1,6 +1,6 @@
 import ColorThief from 'colorthief';
 
-export const getImageMetadata = (imageUrl: string) => new Promise<Record<string, string | number>>((resolve, reject) => {
+export const getImageMetadata = (imageUrl: string) => new Promise<Record<string, string | number | [number, number, number][]>>((resolve, reject) => {
   const image = new Image();
   image.addEventListener('load', async (imageLoadEvent) => {
     const thisImage = imageLoadEvent.currentTarget as HTMLImageElement;