syscall/js: add the Value.Truthy method

Truthy returns the JavaScript "truthiness" of the given value.  In
JavaScript, false, 0, "", null, undefined, and NaN are "falsy", and
everything else is "truthy".

Fixes #28264

Change-Id: I4586f98646c05a4147d06a7c4a5d9c61d956fc83
GitHub-Last-Rev: 649b353ebc23b09d840faf927a2eeca41ee164bf
GitHub-Pull-Request: golang/go#28358
Reviewed-on: https://go-review.googlesource.com/c/144384


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