[whatwg/storage] Add obtain a storage key (#93)

This can be used by APIs that share keying logic with storage, such as BroadcastChannel and shared workers.

Also add and export create a storage shelf so HTML can more cleanly clone sessionStorage data as part of https://github.com/whatwg/html/pull/5560.

Closes #92.


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<a href="https://whatpr.org/storage/93.html" title="Last updated on May 20, 2020, 11:20 AM UTC (bd7a465)">Preview</a> | <a href="https://whatpr.org/storage/93/42510ac...bd7a465.html" title="Last updated on May 20, 2020, 11:20 AM UTC (bd7a465)">Diff</a>
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  https://github.com/whatwg/storage/pull/93

-- Commit Summary --

  * Add obtain a storage key

-- File Changes --

    M storage.bs (59)

-- Patch Links --

https://github.com/whatwg/storage/pull/93.patch
https://github.com/whatwg/storage/pull/93.diff

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