Make LandmarkManager a global singleton

[devongovett]

This ensures that there is only ever a single LandmarkManager instance on the page, even if there are multiple copies or different versions loaded. This is necessary so their global keyboard listeners do not conflict.

It is implemented by attaching the current instance to the document under a hidden symbol. If multiple different versions are loaded, the newest one replaces the older ones. We must ensure that the minimal API in the LandmarkManagerApi and Landmark interfaces never contain breaking changes so that registering landmarks from an older version of useLandmark against a newer version of LandmarkManager continues to work.

[devongovett]

Discussed in slack.

[rspbot]

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## API Changes

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