Add yarn tsd test step + small theme type tweaks

Overview

Adds yarn tsd to the yarn test script so we can actually leverage the index.test-d.ts in CI (or if yarn test is run manually). I added some additional tests and intentionally changed a type to {} to fail the build on my fork: https://github.com/brandongregoryscott/evergreen/runs/4819588995?check_suite_focus=true
Updated ci.yml so that the build steps will run on any branch push, not just master. If workflows are enabled on your fork, this should give you some quicker feedback on whether the CI build will pass on a pull request.
The DefaultTheme.components.<Component>.baseStyle type should have been a Partial to match appearances, sizes, and the Theme baseStyle type.
Finally, per request, I widened the colors theme property and Colors type to include string[]. We don't have any use-cases for it that I know of, but it doesn't clobber any of the typing as far as I can tell, so it should be safe to use.

Screenshots (if applicable)

Documentation

segmentio / evergreen