`WatchEvent` now follows notify v8's format: the event kind is flattened
into the event, with `type` holding the top-level kind and `kind`/`mode`
refining it, e.g. `{ type: 'modify', kind: 'data', mode: 'content' }`
instead of `{ type: { modify: { kind: 'data', mode: 'content' } } }`.
`attrs` is now typed as `WatchEventAttributes` and its `flag` is
`rescan` instead of `Rescan`.
