Add support for parsing ndjson

[dekelpilli]

I've recently come across a problem that would be more conveniently solved if I could parse ndjson strings within VRL. This could also extend to concatenated JSONs (e.g. {"a":1}{"a":2}), or JSONs with a delimiter other than newlines.

I'm not sure exactly what would be the best approach for this feature would be, but I guess it would likely be one of the following:

s = s'{"a":1}\n{"a":2}\n0'

parse_json!(s, format: 'ndjson') # [{"a":1}, {"a": 2}, 0]

# or 

parse_json_stream!(s, delimiter: '\n') # [{"a":1}, {"a": 2}, 0]
parse_json_stream!(s'{"a":3}{"a":4}0') # [{"a":3}, {"a": 4}, 0]

You can achieve something similar with the split +parse_json function, but that doesn't fail fast on JSONs across lines: {"a":"sentence"}\n{"a":"sentence\nwith\nnewlines"}, and doesn't help with parsing JSON streams with no delimiter (e.g. {"a":1}{"a":2})