Add prerelease flag to download

[dpad46]

What are you trying to accomplish?

Add a prerelease flag to thor cldr:download to quickly test prerelease versions of cldr from sources such as this one.

What approach did you choose and why?

Prerelease versions of cldr contain needed files under production/, which differs from the directory structure of released versions. This means that in order to test them, they must be manually downloaded and parsed to vendor/.

As a solution, a separate download#download_prerelease function has been added that only extracts production/ from a given source, such that the files extracted to vendor/ result in the expected directory structure.

What should reviewers focus on?

Prereleases in https://github.com/unicode-org/cldr-staging/releases are very large (~1.5gb). In the future it would be nice to find a way to only download needed files as the large majority of the compressed file consists of documentation.

The impact of these changes

By regularly testing prerelease versions of cldr, preparations for upcoming changes can be made, allowing for quicker upgrades to released versions.

Testing

Manually checked that file is properly extracted to vendor/, and that the extracted files are used as expected.

Checklist

• [ ] I have added a CHANGELOG entry for this change (or determined that it isn't needed)

[movermeyer]

Prereleases in https://github.com/unicode-org/cldr-staging/releases are very large (~1.5gb)

I've added a comment to this issue in CLDR's bug tracker.