Modify BroadcastChannel and SharedWorker method names

[aamuley]

Based on #31 (comment) the storage handle's BroadcastChannel() and SharedWorker() method definitions overlap with the storage object constructor names which is unideal. This updates the spec and explainer to prefer createBroadcastChannel() and createSharedWorker() instead.

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[aamuley]

FYI this PR is failing CI because of this WebIDL issue when we define requestStorageAccess(StorageAccessTypes types) which is unrelated to the actual PR changes.

We would not want to mark the StorageAccessTypes dictionary as optional so we can consider exempting the rule for this case or having {cookies: true} be passed in as a default down the line. Can decide down the line when the spec is actually ready to be updated.

[aamuley]

cc @annevk @bvandersloot-mozilla @cfredric

[annevk]

Would be good for @domenic to review this too as he spotted this first.