feat: add printer package

[Zaba505]

Description

Just like the go package found in Go's standard library, a printer package would be very useful. Both for debugging/testing and for use in gqlc/gqlc.

API

type Config struct {
  Indent string
  UseComma bool
  Comments bool
}

// Fprint "pretty prints" the an AST node to output
func (Config) Fprint(output io.Writer, fset *token.DocSet, node interface{}) error

// FromIntrospection "pretty prints" introspection query results to GraphQL IDL.
func (Config) FromIntrospection(output io.Writer, fset *token.DocSet, src io.Reader) error