Include change event type in `watchChange` calls

[zakcutner]

Thanks for fixing #811 so quickly! As a follow-up issue while trying to use the Windi CSS rollup plugin, I noticed that WMR doesn't pass through a change argument (like Rollup does) for watchChange calls. This causes the plugin to error since it is expecting to read this argument but it does not exist.

I cloned WMR and confirmed that simply adding { event: 'update' } to the watchChange call fixes this issue (WMR seems to only listen for update events in any case). However, I didn't want to make a PR in case it's also desired handle creation/deletion events like Rollup does? If so, I'm not sure how this would affect the other logic that runs on file system changes.