test: add storybook vrt setup for styled-react

[joshblack]

Closes https://github.com/github/primer/issues/6074

Changelog

New

• Add storybook support to styled-react
• Add initial button stories for VRT in styled-react
• Add new utility, getStories, to our e2e test-helpers to help with more easily getting stories from a storybook source file

Changed

• Update our VRT setup to use two storybook URLs, one for react and one for styled-react

Removed

Rollout strategy

• [x] None; if selected, include a brief description as to why

This is a change to our test strategy for styled-react

[changeset-bot[bot]]

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Latest commit: 7d532324a62f7d4a5650a265cfcc2a0f02fab157

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