syscall/js: extend ValueOf to support arrays and objects

syscall/js: extend ValueOf to support arrays and objects

This commits adds []interface{} and map[string]interface{} as quick
ways to create JavaScript arrays and objects. They correspond to the
JavaScript notations [...] and {...}. A type alias can be used for
a concise notation.

Change-Id: I98bb08dbef1e0f3bd3d65c732d6b09e1520026ba
Reviewed-on: https://go-review.googlesource.com/126855
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>