LWG parts of P2996R13 Reflection for C++26

[Eisenwave]

Closes #7930 once the core part is also complete.
Also closes cplusplus/papers#1668.

[jensmaurer]

Forward-looking comment for the library part:

The section labels for the library should be "meta.refl.*" (because shorter without loss of expression) and for the <meta> header, it should be "meta.syn" (because globally consistent where we did not screw up).

Also, "define_aggregate" could maybe become "def.aggr".

[frederick-vs-ja]

I think we should add // see \ref{initializer.list.syn} for consistency. Currently, such comment is added whenever <initializer_list> is included in the synopsis.

[tkoeppe]

@Eisenwave Do you already want review feedback, or is this still in "draft"?

[Eisenwave]

@tkoeppe you can at FIXMEs and TODOs where I'm unsure how to proceed and give me feedback on those; it's not ready for a full review though. I will progress more towards the end of the week on this.

There's also #7987 if you want to sink a few hours or days into reviewing a PR.

[tkoeppe]

Thanks!

[Eisenwave]

Hmm, it has occurred to me that this is a pretty big change.

Everybody expected it and no one is surprised, but when you're 2000 3000+ lines into the diff, it really sinks in ...

[katzdm]

2000 3000+

I was gonna point it out if you didn't..