They now additionally take a direction in which the simulated item usage
should be facing.
Also allow the item parser to parse "hand" and "offhand" as items, to
allow platforms with NBT item support return items with NBT (since
parsing is a slightly more complex task).
All patterns now have suggestions, including recursive patterns.
Suggestions will suggest blocks and block states.
All masks now have suggestions, though mask intersections are not
yet supported due to issues with quotes strings.
EntityRemover and ItemFactory now also have completions, as well
as all RegistryConverters (though I am unsure how many are actually
used).
Also use paper's AsyncTabComplete event, if available.