POC: Use park_cursor to leave cursor at document end

[smheidrich]

Proof of concept to illustrate how to use RustTokenizer's proposed park_cursor method¹ to avoid the "overconsumption" issue from #30 / smheidrich/py-json-stream-rs-tokenizer#47. Feel free to edit or open a new PR altogether with better ideas, this is really just to show how to use it.

There is probably a more elegant solution that doesn't involve the introduction of the level param everywhere just to know when the top-level document ends, but I don't grok the code well enough come up with one. E.g. I initially thought it would be possible to just alter load like

def load(fp, ..., tokenizer=...):
    token_stream = tokenizer(fp)
    ...
    base = StreamingJSONBase.factory(token, token_stream, persistent)
    for thing in base:
        yield thing
    if getattr(token_stream, "park_cursor", None):
        token_stream.park_cursor()

which is similar to how you did it in the minimal example in smheidrich/py-json-stream-rs-tokenizer#47, but I guess that wouldn't work because people could no longer call e.g. persistent on the immediate result of StreamJSONBase.factory...

In any case, this requires smheidrich/py-json-stream-rs-tokenizer#50 to be merged to actually work but I think it makes sense to only merge that once everything has been decided on on this end.

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¹ Still open to better names or turning it into a context manager __exit__ if that makes sense.