add @types/testing-library__dom as a dep

Author: Kent C. Dodds

package.json

@@ -42,6 +42,7 @@
   "dependencies": {
     "@babel/runtime": "^7.5.5",
     "@sheerun/mutationobserver-shim": "^0.3.2",
+    "@types/testing-library__dom": "^6.0.0",
     "aria-query": "3.0.0",
     "pretty-format": "^24.8.0",
     "wait-for-expect": "^1.3.0"