Improve the ".env" REPL command

[YamiDaisuke]

The REPL supports "commands" in the form of .<command-name> structure, one of the first commands added was env which will print the values store in the current execution environment including inner and outer values.

The current output is not so friendly, particularly for arrays and objects the output can be better. For example:

>>> .env
{
  d: (type: let, value: [100, {"foo" : "bar"}]),
  a: (type: let, value: 100),
  z: (type: let, value: {"env" : "[100, {"foo" : "bar"}]"}),
  b: (type: let, value: {"foo" : "bar"}),
  outer: nil
}

Could be something similar to:

{
  d: (
    type: let, 
    value: [
      100, 
      {
        "foo" : "bar"
      }
    ]
  )
  .....
}