Added support for NatSpec in the UDVT definition AST node

[veniger]

Added NatSpec documentation to the User Defined Value Type definition AST node
Partially fixes: #12295

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[nikola-matic]

Looks good. @cameel please take a final look.

[cameel]

I gave this only a brief look, but I don't see anything outright wrong so I'm fine with merging when @nikola-matic approves.

I only have some suggestions as to tests/docs (see below). Also, you have some stray empty lines in the changelog.

[cameel]

Not sure @title and @author make sense here. They were meant for contracts. But I think we already allowed them for structs and enums so I guess for consistency it's better to allow them here too.

The table listing what is usable where should be updated though: https://docs.soliditylang.org/en/latest/natspec-format.html#tags

Also, is @custom allowed? The docs say it's allowed everywhere, but if it is, it would be better to include it in the test to show that.

[gabrielkrell]

Bumping, I would like this. UDVTs render pretty poorly in my documentation without being able to annotate them in any way.