add typescript definition file

index.d.ts

@@ -0,0 +1,4 @@
+export default class ImageCropElement extends HTMLElement {
+  src?: string
+  loaded?: boolean
+}

package.json

@@ -4,9 +4,11 @@
   "description": "Select area for cropping an image. This does not actually crop.",
   "main": "dist/index.umd.js",
   "module": "dist/index.esm.js",
+  "types": "index.d.ts",
   "files": [
     "dist",
-    "index.css"
+    "index.css",
+    "index.d.ts"
   ],
   "scripts": {
     "clean": "rm -rf dist",