Octicon → StyledOcticon, export, tests

[shawnbot]

Fixes #223 based on @broccolini's suggestion to call it StyledOcticon. Here's how you use it:

import {StyledOcticon} from '@primer/components'
import {X} from '@githubprimer/octicons-react'

export default () => <StyledOcticon mx={2} icon={X} />

👀 See the docs in action

[vercel]

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[emplums]

🙌 Beat me to it! Looks good!