Replace boost::variant and tagged unions with std::variant

[ranisalt]

Pull Request Prelude

• [x] I have followed proper The Forgotten Server code styling.
• [x] I have read and understood the contribution guidelines before making this PR.
• [x] I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

Changes Proposed

An effort to reduce the amount of code in headers and remove a dependency, uses std::variant to hold values that can have varying types. This is used for item attributes.

Added a tfs::visitors helper class to avoid writing verbose classes when using std::visit

[ranisalt]

Apple Clang is too handicapped for this