Gather built files sizes and display them in review app

[romaricpascal]

Part of #3279

What

Collect the size of the files built in dist as well as package/govuk,
and display them in the review app ¹.

Ideally this would work as a two step thing so the sizes may be displayed in other ways (like a comment on a PR) or stored somewhere more permanent.

If quick enough to add, it could be neat to also collect Gzip and Brotli size (the gzip-size and brotli-size packages may be helpful there).

Why

This is one of the steps towards keeping an eye on the size of the files we generate. It'll let us check if our changes introduce an unexpected variation of size in the built files.

Who needs to work on this

Developers

Who needs to review this

Developers

Done when

• Collect size of files built in dist and package/govuk into JSON
• Render collected sizes on a page in the review app²
• Either:
  • Collect Gzip and Brotli sizes
  • or
  • Create an issue for future collection of Gzip and Brotli sizes

Footnotes

1. For humanizing the file sizes, the filesize package may come handy. ↩

2. This will/may call for building dist and package on heroku, which we might be able to speed up by running in parallel, using something like npm-run-all. ↩