Feature Request: further JSON5 support

[matty0ung]

It would be really neat if Boost.JSON could support some or all of the JSON5 features (via additional flags on parse_options).

We already have allow_comments and allow_trailing_commas, but it would be great to have, say, allow_json5_strings to allow that strings may:

• be single quoted;
• span multiple lines by escaping new line characters;
• include character escapes.

[grisumbras]

I can see the benefits of this, but on the other hand these aren't the only JSON5 features that we are missing, and the other features to me seem like they have significant implications for the parser. The features requested in this issue itself also may have sizable impact on the parser. So I can't promise this will be implemented. But I'll keep the issue for future experimentation.

[matty0ung]

Thanks! For me personally, multi-line strings would be very useful, but I appreciate it might be a non-trivial change.