Add apply*() functions (visitor pattern)

[falbrechtskirchinger]

This has been factored out of #3446 but builds upon that PR and needs to be rebased.

Originally, I was looking for a workaround to be able to use std::ranges::sort() with basic_json. std::ranges::sort(j) fails to compile because basic_json doesn't (and can't) satisfy the std:random_access_iterator concept. This PR is the product of this search and adds 3 variants of visitor-style apply*(Fn, Args...) functions to basic_json which invoke a given callable (Fn) with the stored JSON value and any additional arguments (Args...).

(Describe functions, etc.)

To Do:

• [ ] Exceptions are placeholders. Throw appropriate type, maybe re-word messages, and pick (and document) an ID number.
• [ ] Complete unit tests.
• [ ] Update documentation.
• [ ] Look at discussion #3779. Potential use case?

[coveralls]

Coverage decreased (-0.02%) to 99.983% when pulling 04b4d79ec22b3b08534bc3191da45232f15a5993 on falbrechtskirchinger:apply into b205361d8652759b6d850a37b227c8d57ee19005 on nlohmann:develop.