allow filtering pull requests by if the author is a bot (author_is_bot:<bool>)

[thesuperzapper]

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I think we should be filter pull requests (and issues) by if they were created by a bot.

This is a big deal for large projects that use bots like dependabot, as they can create hundreds of PRs and make it hard for feature / bug-fix PRs to get attention from maintainers.

Once some kind of author_is_bot:<bool> filter is added, I think this filter should be highlighted in the default "open pr" view, perhaps as a checkbox: