fix: dont externalize mermaid (#1033)

[![PR App][icn]][demo] | Fix RM-XYZ
:-------------------:|:----------:

## 🧰 Changes

Don't externalize `mermaid` for the node build.

In `mdx-renderer` it's dynamically imported, so it doesn't usually get
executed. If we externalize it, we either have to specifically include
it in the execution environment, or configure rollup to inline it. The
former is an unnecessary hassle, and the latter appears to execute the
code immediatelly and slow down the build. So lets go back to bundling
it, but dynamically importing it.

## 🧬 QA & Testing

- [Broken on production][prod].
- [Working in this PR app][demo].


[demo]: https://markdown-pr-PR_NUMBER.herokuapp.com
[prod]: https://SUBDOMAIN.readme.io
[icn]:
https://user-images.githubusercontent.com/886627/160426047-1bee9488-305a-4145-bb2b-09d8b757d38a.svg

webpack.config.js

@@ -119,9 +119,6 @@ const serverConfig = merge(getConfig({ target: 'node' }), {
     filename: '[name].node.js',
   },
   externals: {
-    '@readme/variable': '@readme/variable',
-    '@tippyjs/react': '@tippyjs/react',
-    mermaid: 'mermaid',
     react: {
       amd: 'react',
       commonjs: 'react',