Add `.clang-format` file to repository

[Arsenic-ATG]

The code base uses GNU coding convention, most of which can easily be enforced using clang-format, the project should contain a .clang-format containing the clang format's configuration for GNU coding style.

fortunately clang format supports GNU coding standards as one of it's presets so we can simply use clang format to generate that file for us using a command like this :

$ clang-format -style=gnu -dump-config > .clang-format