Add behavior and generic hook for anchored positioning.

[T-Hugs]

This PR adds a generic behavior for positioning a floating element next to an anchor. For details, see docs.

Play with the behavior: https://codepen.io/team/GitHub/pen/OJbPKNZ

See the rendered documentation. Note: this was intentionally not added to the documentation nav page since we decided to leave these behaviors undocumented until they are used in components.

Closes#1079

Merge checklist

• Added or updated TypeScript definitions (index.d.ts) if necessary
• Added/updated tests
• Added/updated documentation
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge

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Latest commit: 53a9703

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🔍 Inspect: https://vercel.com/primer/primer-components/EVNXEHNACY2xaj7UCuVCHCZwiHQR
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[emplums]

Is there any way we can get this demo in the page? I'm worried about this code not staying up to date with the code in the actual useAnchoredPosition hook. Either that or import the hook from the PRC package in codepen

[emplums]

This is really great! Just left a few more comments otherwise I think this is ready 2 go (pending more reviews)! Nice work! 🚀