[native] Create Flow ExpoBarcodeScanner libdef

Summary:
To use certain types when writing the BarCodeScanner integration, it'll probably be useful to have the types available (not entirely necessary, so this can always be abandoned / revisited at a later point if it takes too much time).

Mainly used the types in the flow-typed repo as inspiration alongside the actual typescript file from expo. I did only include the methods in the class that I planned to use, but if it's better to include them all I can do that

Depends on D8770

Test Plan: Confirmed that I can now use these types on native. Also ran flow and no errors were present

Reviewers: atul, ginsu, tomek, varun

Reviewed By: atul

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D8771