Add msgspec.json.format function

[jcrist]

This takes an existing JSON document and reformats it, adding/removing whitespace as needed. The formatting is done efficiently, for common use cases the overhead should be negligible.

Demo:

In [1]: import msgspec                         

In [2]: msg = b'{"a":[1,2,{"x":2}]}'  # an existing JSON message

In [3]: print(msgspec.json.format(msg).decode())  # pretty print
{                                              
  "a": [                                       
    1,                                         
    2,                                         
    {                                          
      "x": 2                                   
    }                                          
  ]                                            
}                                              

In [4]: print(msgspec.json.format(msg, indent=4).decode())  # configurable indent size
{                                              
    "a": [                                     
        1,                                     
        2,                                     
        {                                      
            "x": 2                             
        }                                      
    ]                                          
}                                              

In [5]: print(msgspec.json.format(msg, indent=0).decode())  # indent 0 only adds spaces
{"a": [1, 2, {"x": 2}]}

In [6]: formatted = msgspec.json.format(msg)

In [7]: print(msgspec.json.format(formatted, indent=-1).decode())  # indent < 0 minimizes
{"a":[1,2,{"x":2}]}

Fixes #219.