Add operator< to , to allow use as a map key

[khiner]

This is a replacement for #3667

After looking at how extensive the commit for the other equality operators is, I see how much this PR needs that's not here (tests/docs/etc). I'm more than happy to cargo code/copy-paste and edit based on that commit to make this one look just like that. Lmk if that would be of value and actually helpful and not just a burden to you vs doing it yourself.

[coveralls]

Coverage remained the same at 100.0% when pulling 9ebfeb7 on khiner:develop into 7b6cf59 on nlohmann:develop.

[falbrechtskirchinger]

I'm not too happy with the current state of the operator==/!= string overloads, but not having them means we're not compatible with v3.10.5.
Technically, the same is true for the other operators, but I'm hoping that those are rarely used in practice.

Since you're only looking for map support, maybe we should limit this PR to comparing two JSON pointers for less-than and be done.

If @nlohmann doesn't object, I'd also skip the documentation for now. It's actually easier for me to copy and adjust the documentation from basic_json/json_pointer, once all operators (including <=>) are done.

Also, note that I'm considering one more (small) change to the existing operators. There shouldn't be any merge conflicts or only minor ones. (Waiting on feedback before I proceed.)

[falbrechtskirchinger]

I added the bare minimum to get map support into 3.11.2. Hope you don't mind. (#3685)

I'll add documentation when I add the remaining operators if Niels allows it.

[khiner]

Thanks @falbrechtskirchinger !