Specify a standard version of `clang-format`

[wbernoudy]

Since we expect the C++ code to be formatted with clang-format, but there are inconsistencies between different versions, we should specify the version one is expected to use when doing development.

[arcondello]

We do not currently enforce clang-formatted code. We could consider doing so as well.

[wbernoudy]

Agreed! For now at least, it would be nice to just have a note in the readme.

[arcondello]

I don't think this is easy.

The ecosystem as a whole doesn't have a lot of consensus.

• VSCode uses 18.1.7 (I think?) https://github.com/microsoft/vscode-cpptools/blob/1e89750b0bb9629ba0262a6eb44d1ab41eb77d51/Extension/CHANGELOG.md?plain=1#L137
• apt install clang format pulls in 14.0.0
• The LLVM source tree itself doesn't seem to be well formatted, albeit as of March 2022.

For internal repos, I am reasonably comfortable enforcing a version. But for public repos like this, where we want to encourage contribution from users, I don't think it's reasonable to require folks to install a specific version of clang simply to make a PR against this repo.

We could format as part of CI, but that comes with its own issues.