js: reimplement subprocess using the subprocess command (match 548ef078)

Semantics changes are the same as at 548ef078 .

Also, the previous C implementation returnd a string for the `stdout`
value, but stdout of the subprocess command is MPV_FORMAT_BYTE_ARRAY
which js previously didn't support, so support it too (at pushnode)
by returning it as a string - the same as the lua code does.