The TOML editor offered nothing when typing a date, leaving the
yyyy-mm-dd format undiscoverable until a parse error. Suggest today
(a full, easy-to-edit date) and the current-year prefix on empty
date values and fresh reestimates elements, backing off once a date
is being typed.
Given a source and caret offset, suggest block headers, the keys valid
for the current block (excluding ones already written), status enum
values, and the feature names referenced by a milestone's requires.
Mirrors the schema in parse.ts.
