Upgrade to 1.0.0-rc853

Concerns: pushword/admin, pushword/admin-block-editor

The show-more calls accept named arguments

{{ startShowMore(showMoreExtraClass: 'mt-8') }} is read as a collapsible group, and is what the editor writes when the group carries a class but no anchor. A call naming an argument the function does not have stays a Raw block instead of being re-exported without it. Nothing to do.