Define CAR v2 as a separate go module

Since we are releasing a new major version, define a new go module for
the CAR v2 implementation, following the recommendations here:
- https://blog.golang.org/v2-go-modules

The module is versioned as go 1.15 to maintain compatibility as pointed
out by comment here:
- #75 (comment)

v2/go.mod

@@ -0,0 +1,3 @@
+module github.com/ipld/go-car/v2
+
+go 1.15