Add ERC-2981 royalties facet

[aapsi]

Add ERC-2981 royalties facet that exposes royaltyInfo with default and per-token overrides. Marketplaces can query royalties directly on the diamond.

[aapsi]

@mudgen Can I please work on it if it aligns with the scope?

[mudgen]

Hi @aapsi. Yes, you can work on this. But you need to follow these points:

1. Follow the Solidity feature ban exactly.
2. Follow the Design principles of Compose.
3. You can't implement the EIP-165 part right now, so don't implement that part.
4. Look at the ERC20 or ERC721 implementations in Compose and make sure your code style is consistent with them -- for example parameters to functions are prefixed with an underscore "_". Look at how LibERC20 and LibERC721 are written and do the same.
5. Make it compatible with OpenZeppelin's implementations. That means, for example, if the OpenZeppelin logic emits a certain error or event in a particular case, or reverts in a certain case, then you do the same thing in that case with the same error message or event, etc. Just want things to work the way is currently expected. But the way we write it is different.

I plan to write more contributor documentation to make more clear the style guide and other specifics.

[aapsi]

Hi @mudgen
Thank you for the detailed and very helpful guide. I will get to work

[aapsi]

Hi @maxnorm @mudgen
Will be finalising the PR this weekend.

[maxnorm]

This issue was closed by this PR Add ERC-2981 NFT Royalty Standard implementation #126