FIX: add "main" field to package.json

fix #265

Author: yanick

package.json

@@ -2,6 +2,7 @@
   "name": "@testing-library/svelte",
   "version": "0.0.0-semantically-released",
   "description": "Simple and complete Svelte testing utilities that encourage good testing practices.",
+  "main": "src/index.js",
   "exports": {
     ".": {
       "types": "./types/index.d.ts",