feat(button): add <rh-button>

[bennypowers]

What I did

1. Add rh-button based on BaseButton from pfe
2. Uses the new ElementInternals API to remove need for slotted button

Closes #370

Testing Instructions

1. npm start

And check the DP demos:

4. Main demo
5. Color context demo
6. Form control demo

Notes to Reviewers

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🦋 Changeset detected

Latest commit: 9593006

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

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@rhds/elements	Minor

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[bennypowers]

4747746 does two things:

1. Initial draft of FACE for button
2. Extend from LitElement

At the moment I'm leaning towards keeping step 2, even if we implement FACE in pf-button, because the styles are so much cleaner and simpler. It's still 'downstream' from pfe on account of helpers and controllers, and we may upstream the face stuff into controllers and whatnot, but consider the difference between rh-button.css and BaseButton.scss.

[bennypowers]

NB: there are some build issues here around class inheritance. probably will be resolved by #725

[marionnegp]

Dark theme's secondary, secondary danger, and tertiary have a 3px border width with focus. Should be 2px only. Not sure what in the CSS is causing this.

[bennypowers]

@marionnegp @nikkimk ok hopefully that should do it 🤞

4. Main demo
5. Color context demo
6. Form control demo

[nikkimk]

Lexicon Grifting Technical Mumbles

[bennypowers]

@marionnegp yours is the last blocking review... At your leisure

[marionnegp]

A couple changes, but everything else looks good!