refactor(Flash): move files to folder, add e2e tests and stories

[joshblack]

Parent issue: https://github.com/github/primer/issues/1488

Add storybook stories for Flash

Checklist

• Ensure your chosen component has a component directory in the src directory
• Ensure the component has a root story file in the component directory
• Ensure that the root story file exports a Default story
• (Optional) Create a Playground story
• Move all other stories for the component into a .features.stories.tsx file
• Opt the component into storybook tests
• (Optional) Create visual regression tests for the component

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[github-actions]

size-limit report 📦

Path	Size
dist/browser.esm.js	89.68 KB (-0.01% 🔽)
dist/browser.umd.js	90.24 KB (-0.01% 🔽)

[broccolinisoup]

Should we use MetaComponent and StoryComponent here? storybookjs/storybook#16650 (comment)
Or was there any reason we used this version?

[joshblack]

@broccolinisoup great catch, I've just been using existing templates but I think you're dead-on that we should be using MetaComponent and StoryComponent.

cc @langermank just a heads up for above 👆, it also seems to address props like as and forwardedAs causing issues on argTypes 🥳