fix: fix rendering issue in Firefox

This fix moves the responsibility of the rendering into the extension, rather than the mule-preview
plugin. An issue was uncovered  where the latest version of React (16.9.0) does not work in Firefox
browser extensions due to a change to the scheduler (facebook/react#16606)
so React was downgraded. After those fixes were implemented, the issue with rendering only working
correctly the second time the extension was triggered was fixed.

fix #72

.babelrc

@@ -1,6 +1,7 @@
 {
   "presets": [
-    ["@babel/preset-env", { useBuiltIns: "usage", corejs: 3 }]
+    ["@babel/preset-env", { "useBuiltIns": "usage", "corejs": 3 }],
+    "@babel/preset-react"
   ],
   "plugins": [
     "@babel/plugin-syntax-dynamic-import",

.eslintrc.json

@@ -1,5 +1,9 @@
 {
-  "extends": ["eslint:recommended", "plugin:prettier/recommended"],
+  "extends": [
+    "eslint:recommended",
+    "plugin:react/recommended",
+    "plugin:prettier/recommended"
+  ],
   "env": {
     "browser": true,
     "commonjs": true,