Update fidlviz for v2 wire format

This CL adds a checkbox that enables use of the v2 wire format. It is
checked by default.

Change-Id: I843af7e71d922ed7e3f0915eacca5d04212e0043
Reviewed-on: https://fuchsia-review.googlesource.com/c/fidl-misc/+/584462
Reviewed-by: Benjamin Prosnitz <bprosnitz@google.com>