internal/export/idna: implement Context J rules

All tests now pass.

Also includes modifier information in the table.

Updates golang/go#17268

Change-Id: I7a7a9cdad9a654e0826617925dc9cdf0537c217e
Reviewed-on: https://go-review.googlesource.com/31273
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>