Bazel: Improve support for bzlmod

[Vertexwahn]

The Bazel build system now supports a new way of handling 3rd party dependencies, such as JSON for Modern C++. For this new way, a MODULE.bazel file is needed - in the long term, the WORKSPACE.bazel file will not be needed anymore and will be deleted in the future.

[coveralls]

coverage: 100.0%. remained the same when pulling 55e16a4 on Vertexwahn:bazel-support into 5d27543 on nlohmann:develop.

[nlohmann]

Do I really need to add more files just to support Bazel?

[Vertexwahn]

@nlohmann In the long term MODULE.bazel will replace the WORKSPACE.bazel file. Maybe its to early for this switch. If you do not want to have more Bazel files in your repo we can simply close this PR and I will do the replacement of WORKSPACE.bazel by MODULE.bazel when this is the main way of Bazel to handle 3rd party deps...

[nlohmann]

@nlohmann In the long term MODULE.bazel will replace the WORKSPACE.bazel file. Maybe its to early for this switch. If you do not want to have more Bazel files in your repo we can simply close this PR and I will do the replacement of WORKSPACE.bazel by MODULE.bazel when this is the main way of Bazel to handle 3rd party deps...

Ok.