Bump @babel/plugin-proposal-nullish-coalescing-operator (#30)

Bumps [@babel/plugin-proposal-nullish-coalescing-operator](https://github.com/babel/babel) from 7.4.4 to 7.7.4.
- [Release notes](https://github.com/babel/babel/releases)
- [Changelog](https://github.com/babel/babel/blob/master/CHANGELOG.md)
- [Commits](https://github.com/babel/babel/compare/v7.4.4...v7.7.4)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

package.json

@@ -36,7 +36,7 @@
     "@babel/core": "^7.2.2",
     "@babel/plugin-proposal-class-properties": "^7.7.4",
     "@babel/plugin-proposal-decorators": "^7.2.2",
-    "@babel/plugin-proposal-nullish-coalescing-operator": "^7.4.4",
+    "@babel/plugin-proposal-nullish-coalescing-operator": "^7.7.4",
     "@babel/plugin-proposal-optional-chaining": "^7.2.0",
     "@babel/preset-env": "^7.2.2",
     "@babel/preset-react": "^7.0.0",