Supersedes https://github.com/nostr-protocol/nips/pull/524

# Purpose

Content moderation involves labeling content. In https://github.com/nostr-protocol/nips/pull/459, @gruruya proposed expanding NIP-56 to encompass labeling. I chose to create a new NIP rather than update NIP 56 because backwards compatibility would have been difficult to maintain, and because the purpose of the NIPs are slightly different (see below).

In https://github.com/nostr-protocol/nips/pull/457, @s3x-jay and @rabble proposed a nomenclature and more comprehensive mechanism for content reporting/labeling. The consensus seems to be that a nomenclature is not a good idea, but there were some good ideas to include.

In https://github.com/nostr-protocol/nips/issues/522#issuecomment-1546286149 I realized that the same labeling mechanism could be used to leave reviews for relays, which I am eager to add to Coracle so people can start to find a better relay selection.

This PR may also render https://github.com/nostr-protocol/nips/pull/46 obsolete.

# Justification

Maybe this will seem like I'm trying to make a single NIP do too many things. But reviews, labels, and reports all have two things in common: they refer to an object, and are an expression of someone's opinion. In each case, there is a value judgment involved in assigning the label, involving both imprecision and uncertainty. Expressing this opinion using either a mark, or content, or both, allows either a machine or human to make a judgment call as needed.

One nice thing I'd also like to point out about this PR is it allows for associating any two entities (e/p/r/t) in nostr, and searching those associations. This gives us the beginning of graph-database-like functionality that can form the basis for both WoT and topical moderation and recommendations.

Confidence/quality may seem like the same thing, but they're two different axes along which to measure certainty. The first is subjective (the label author may not be 100% sure), the second objective (the label may not perfectly fit). This has been suggested in a few places, but was best executed by guide.newfounding.org (unfortunately their site is no longer available).

# Compatibility

This PR is similar to NIP-56, but is focused on distributed "moderation" (allowing users to filter notes based on their preferences) rather than centralized moderation implemented by relays and clients directly as required by law or app store policy.

# Implementation

None of this is implemented, but my main motivation of drafting this PR is so I can add relay ratings and reviews to Coracle soon.

https://github.com/nostr-protocol/nips/pull/532