syscall/js: rename Callback.Close to Release and expose Callback.Value

This makes Callback more in line with TypedArray. The name "Release" is
better than "Close" because the function does not implement io.Closer.

Change-Id: I23829a14b1c969ceb04608afd9505fd5b4b0df2e
Reviewed-on: https://go-review.googlesource.com/121216


Run-TryBot: Richard Musiol <neelance@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>