[Feature] onRebuild result should contain metafile if options ask for it

[dzearing]

When using watch mode, the metafile is returned from the initial build result, but not in the results from onRebuildwhen things change. I'd like to access the bundle metadata if possible when rebuilding (assuming metafile: true is set in options.) This would enable some interesting scenarios such as live updates on dev tools for monitoring bundle size, chunk changes over time, etc.

[evanw]

Thanks for the report. This was not intentional, and was just an oversight. Will fix.