* feat(parser): export parser.Children
As parser was moved to internal/parser, it is not possible to import it into
external projects anymore.
However, parser.Children() is handy to use in static analysis tools, so it is
worth exporting. This does that by adding a stub package
`github.com/google/go-jsonnet/parser` that wraps the internal function.
Signed-off-by: sh0rez <me@shorez.de>
