fix(node): drop Node 8 support (#576)

Closes #575

BREAKING CHANGE: This release drops support for Node 8. Upgrade to Node 10 or greater.

Author: kentcdodds

Diff for: package.json

@@ -5,7 +5,7 @@
   "main": "dist/index.js",
   "module": "dist/@testing-library/react.esm.js",
   "engines": {
-    "node": ">=8"
+    "node": ">=10"
   },
   "scripts": {
     "prebuild": "rimraf dist",
@@ -86,4 +86,4 @@
     "url": "https://github.com/testing-library/react-testing-library/issues"
   },
   "homepage": "https://github.com/testing-library/react-testing-library#readme"
-}
+}