Add outline and constituent properties. (Resolves #84, #85, #144.)

[nsaunders]

Description of the change

This change primarily adds support for outline, outline-style, outline-color, outline-width, and outline-offset. Unfortunately it supersedes #85 (which regrettably has sat unmerged for years), but hopefully it will be easy to understand why after reviewing the full scope of changes.

Aside from outline properties, the tests that I added (based on examples from MDN) revealed some missing type class instances (see #144).

Finally, I have begun implementing a different architecture for the test suite based on Clay. Hopefully this will make it easier in the future to "copy and paste" their tests if desired. 😉

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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 todo (docs in general)
• Added a test for the contribution (if applicable)