syscall/js: add Value.Delete for deleting JavaScript properties

This change adds the method Value.Delete, which implements
JavaScript's "delete" operator for deleting properties.

Fixes #33079.

Change-Id: Ia5b190240bd59daca48094fcbc32f8d0a06f19d5
Reviewed-on: https://go-review.googlesource.com/c/go/+/197840


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