Spike script to build release with dev dependencies stripped out

[lfdebrux]

First pass at a script that would mean releases don't include dev dependencies, making installation of the kit quicker for all users.

Notes:

• This would affect our ability to use the download zip feature. The zip downloads would no longer be like releases.
• We would also have extra files appearing on each page on GitHub for each release, I don’t think we can tell GitHub not to create a source code zip. So with a build process we would have to be careful about how we communicate to users on the GitHub release page about which file to download.

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Not addressed in this spike: tests of the release script, using the release script for tests

[lfdebrux]

I've created a draft release to share the archive generated from this script, and to show what the release page would look like:

https://github.com/alphagov/govuk-prototype-kit/releases/tag/untagged-f21dbf728816b9509765