USWDS-Elements - add workflow for preview releases

[ethangardner]

We would like a way for people to be able to install and give feedback for the tokens (and elements in the future). We want to publish test-builds as GitHub releases on specific branches and clean them up when the branch is merged.

Requirements:

• The package should be versioned with the branch name and have an obvious version distinction from the tagged production-ready releases distributed on GitHub releases and available for install on NPM.
• The release notes for the test build should be auto-generated and include a message that indicate that it is a test build and should not be used for production applications. It should make a deliberate and clear distinction that indicate the production release.
• It should be installable via zip without having PAT or doing complicated npm link
• test build releases should be cleaned up when the branch is merged
• Preview releases should only be generated for specific PRs
• The workflow should not run on forks of the repository
• The workflow should be able to run on-demand
• It should not interfere with someone who would like to install a production-ready version. e.g. it should not use a custom registry edit

[ethangardner]

@annepetersen I'm laying this out per our discussion. Can you provide the message that the test build should include to differentiate it from a production-ready version?