Glaze v5 Breaking Change Proposals

[stephenberry]

This discussion is to provide a space for writing down API breaking changes for Glaze version 5 that would help bring more clarity to the library.

[stephenberry]

Remove glz::detail namespace around to/from specializations

The to/from specializations are used to write custom serialization code. I feel like the APIs are formalized now and the to/from specialization should be moved out of the detail namespace. This will make C++20 modules export definitions make more sense, as they won't have to be applied to all these detail structs.