Add header only consumption mode

[mborland]

See: https://github.com/boostorg/json#header-only

[pdimov]

This isn't a particularly good or useful idea. It's not practical to have a header-only option for this library. Either it should be always header-only, or always compiled.

That's because now every downstream consumer needs to pick a mode, and they all must pick the same mode. E.g. if JSON uses the library in header-only mode, and my code uses JSON, my code (and all its dependencies) also must use header-only mode. And conversely, if JSON uses compiled Charconv, everyone else must, too.

Also, the above link doesn't actually describe header-only, even though it's called such.