Add a .clang-format

[billy4479]

This is a config file for clang-format, a tool that is already used by many editors and adopted by many projects to format the code and ensure that the style is consistent.
It could even be used as a git hook or in the CI/CD workflow to ensure that new patches respect the coding style.
A config can derive from other preexisting configs (like Google's, LLVM's or Mozilla's) so it's not a big effort to write one.
It is not the number one priority but I think that's a great thing to have.
More information at LLVM's website.