[clang-tools-extra] [clang-tidy] Add modernize-nlohmann-json-explicit-conversions check (PR #126425)

[via cfe-commits]

whisperity wrote:

> It feels like instead nlohmann/json should be the one responsible for providing a "migration path" / tooling for these types of deprecated features.
> 
> Thus I would vote for keeping clang-tidy as a general tool for C++/STL only. 

N.B. we already have `abseil-`, `boost-`, `cppcoreguidelines-`, `fuschia-`, `linuxkernel-`, and `zircon-`.

https://github.com/llvm/llvm-project/pull/126425