Summary: Fixes T13662. Phriction currently passes a map as a "context object", but this code is ancient and predates the modern meaning of a "context object". In modern code, context objects should be real objects. Provide a real object as a context object. We do this by either loading the actual document or constructing a synthetic version of it. Test Plan: - Edited an existing document, observing the preview: - Used a mention rule, saw a preview. - Used `[[ a ]]` and `[[ ./a ]]` absolute and relative reference rules, saw accurate previews. - Edited a new document, observing the preview: - Used a mention rule, saw a preview. - Used absolute/relative references, saw accurate previews. - Grepped for other references to the removed properties (`phriction.isPreview` and `phriction.slug`), found none remaining. Reviewers: 0 Reviewed By: 0 Maniphest Tasks: T13662 Differential Revision: https://secure.phabricator.com/D21709