build: Add `@babel/plugin-proposal-nullish-coalescing-operator` babel plugin

Forces Babel to transplile `??` to ES6 syntax.

babel.config.js

@@ -7,6 +7,7 @@ module.exports = (api) => {
   return {
     presets: [
       ['@babel/preset-env', {
+        include: ['@babel/plugin-proposal-nullish-coalescing-operator'],
         loose: true,
         targets,
         useBuiltIns: false, // Don't add polyfills automatically.

package-lock.json

@@ -21,6 +21,7 @@
         "@babel/core": "^7.14.6",
         "@babel/eslint-parser": "^7.11.3",
         "@babel/eslint-plugin": "^7.11.3",
+        "@babel/plugin-proposal-nullish-coalescing-operator": "^7.14.5",
         "@babel/plugin-transform-react-jsx": "^7.10.4",
         "@babel/preset-env": "^7.14.7",
         "@babel/register": "^7.10.5",

package.json

@@ -40,6 +40,7 @@
     "@babel/core": "^7.14.6",
     "@babel/eslint-parser": "^7.11.3",
     "@babel/eslint-plugin": "^7.11.3",
+    "@babel/plugin-proposal-nullish-coalescing-operator": "^7.14.5",
     "@babel/plugin-transform-react-jsx": "^7.10.4",
     "@babel/preset-env": "^7.14.7",
     "@babel/register": "^7.10.5",