This diff adds the package to package.json, and also introduces a Flow libdef.
Our other React Navigation libdefs were updated for React Nav 6 as part of ENG-336, but since we didn't use Drawer at the time I didn't update that one fully.
The Flow libdef here has SECTION 1 copy-pasted from the other React Nav 6 libdefs, but SECTION 2 is from the old React Nav 5 libdef currently on flow-typed. I'll update SECTION 2 based on the changelog in a subsequent diff, but since it looks like the old libdef for React Nav 5 was fairly incomplete, we'll likely need further updates.