Several areas within blender can benefit a JSON reader/writer library.
Areas like the asset browser, XR and grease pencil.

After looking at the available options we selected nlohmann's JSON for
modern C++ library. It is actively maintained for over 10 years and
flexible.

This patch only adds the header only implementation of the library so it
can be used by different areas. The asset browser project is planning to
add a small abstraction layer so it will be easier to switch between
several different serialization formats. This is currently in
development in D12544.

In cases the abstraction layer can be an overhead and undesired to be used.
In this case the header file can be directly included.