In MLIR dialect, use LLVM's built-in JSON library

[sampsyo]

As of #463, @Mond45 added a really cool implementation of a Bril MLIR dialect. There is a translation to and from "real" Bril in JSON form. These transitions currently depend on the ubiquitous nlohmann/json C++ library as an external dependency.

However, LLVM these days includes its own JSON library. It could be a "fun" project to convert its JSON-interacting components to use that library, thereby eliminating a dependency.