Provide autocomplete also for tagging in the YAML frontmatter section

Using tag hierarchies in the body of a document is easy since tag choices in the hierarchy display automatically as you type. There is no similar behavior for the “tags” attribute in YAML frontmatter, however. It would be very nice to have the behavior there too.

Autocomplete works if you write it like this in YAML:
tags: " #thisisatag #ipickedthisfromautocomplete "

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