### What?
Followup for #47630 , use ts namespace to group MetadataRoute.
`MetadataRoute['xxx']` -> `MetadataRoute.xxx`
### Why?
namespace is convenient to write for these grouped types such as
`MetadataRoute.Robots`. And `MetadataRoute` itself is not actually a
type. So it can be conveniently typed by ts users.
### How?
Closes NEXT-912
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35cf930572