feat: Spinner component

[dgreif]

This is a basic Spinner component which renders a simple spinner SVG with rotation. Besides COMMON system props, the only option is size, which is currently limited to 'small' | 'medium' | 'large'. This matches the behavior of the PVC spinner. Although the docs/tests are all done, I'm marking this as an alpha to start with, just in case we decide to change the type for size.

Closes https://github.com/github/primer/issues/165

Screenshots

Merge checklist

• Added/updated tests
• Added/updated documentation
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge

[changeset-bot]

🦋 Changeset detected

Latest commit: 2eb3d92

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

This PR includes changesets to release 1 package

Name	Type
@primer/components	Minor

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[colebemis]

Looks good! Thanks for adding this, @dgrief. Just needs a changeset.