Add focus style for play/pause button

[dhruvkb]

Fixes

Fixes #148 by @sarayourfriend

Description

This PR adds a ring around the play/pause toggle button when focused. The Tailwind config added in the PR would also be useful in resolving #147.

Technical details

The focus outline is only shown for the focus-visible state. Browser support is good but Safari is the next IE.

Screenshots

The border fades in and out, similar to Gutenberg. Since the button has no rounded corners, neither does the outline.

Checklist

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[krysal]

LGTM