Add support for exporting with indentation

[fm2003]

The current version of json.bqn only supports exporting in a "compact" format:

{"k1":{"sub":10},"k2":20,"k3":[30,40,50]}

This PR adds support for "pretty-printing" the JSON export (i.e. adding line breaks, indentation, and spaces after ':'):

{
    "k1": {
        "sub": 10
    },
    "k2": 20,
    "k3": [
        30,
        40,
        50
    ]
}

The functionality is inspired by dumps from Python's json module (including its limitations, such as not being able change formatting behaviour between objects and lists).