Member visit(P2637)

[libbooze]

P2637 is merged into C++26 draft.

Implementation should not be hard, although a bit more complicated than in std::variantsince I presume we want to allow this functionality in standards that do not support deducing this.

If maintainer is fine with this change I could try to implement it, as mentioned logic does not seem hard beside code duplication due to lack of deducing this.

edit: if maintainer feels functionality for older standards is not worth the trouble then I believe at least we should implement version with deducing this in newer standards to keep variant2 API matching std::variant.