Introduce purs-tidy formatter

[thomashoneyman]

Description of the change
Introduces the purs-tidy formatter. This formatter is a lightly opinionated tool we use to maintain a consistent style in the contrib libraries.

We ordinarily restrict to tools provided by the core or contrib projects. This tool is not, but it was developed and is maintained by core team members, and because it's a formatter it can't block maintenance or release of this library even in the event something goes catastrophically wrong with it.

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Checklist:

• Added the change to the changelog's "Unreleased" section with a link to this PR and your username
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation in the README and/or documentation directory
• Added a test for the contribution (if applicable)