Investigation notes for flag analogous to JSON_BIGINT_AS_STRING

[TysonAndre]

This may or may not be difficult with the dom parser.

Future simdjson C releases may make this easier but this functionality might not be planned for arbitrary-precision integers (only floats) - https://github.com/simdjson/simdjson/pull/1886 is in review pending manual tests and performance testing.

• See my PR comment for a note on how BIGINT might be possible in followup

https://github.com/simdjson/simdjson/issues/167 suggests it's possible with the ondemand parser, but that may be slightly slower than the dom parser?

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https://github.com/simdjson/simdjson/issues/425#issuecomment-883605600 mentions this is supported in the ondemand api

We may want to take the tiny performance hit just to imitate php's behavior for numbers in json_decode()

Note that this is effectively supported now with the On Demand API and documented as such:

simdjson::ondemand::parser parser;
simdjson::padded_string docdata =  R"({"value":12321323213213213213213213213211223})"_padded;
simdjson::ondemand::document doc = parser.iterate(docdata);
simdjson::ondemand::object obj = doc.get_object();
std::string_view token = obj["value"].raw_json_token();

[TysonAndre]

https://github.com/TysonAndre/simdjson_php/tree/wip-ondemand-parser has incomplete changes exploring what would be needed to use ondemand instead.
Some limitations make it a better idea to stay with the dom implementation - this also keeps the bindings simpler:

• Depth isn't checked automatically in ondemand (same for json_pointer)
• Can't iterate over a document multiple times
• Document isn't automatically validated, which it currently is (e.g. for simdjson_key_exists())