[suggestion] Add .clang-format to help fit the existing coding style

[Arpafaucon]

While working on #825 I found useful to spin up this config file to be able to fit the existing coding style.

It's not perfect, but I already goes a long way; maybe you'd be interested ?

Known caveats:

• could not manage to keep small enum on a single line
• you put 2 spaces in ifdefs (instead of the 4 elsewhere), haven't managed to replicate it
• it's not possible to remove the spaces around binary operators Can't see option in clang-format to omit spaces around binary operators llvm/llvm-project#31431

I committed 2 files so you can quickly look at the diff and get a sense of how files are affected

[wolfpld]

How does this work in practice, when you are editing files? Proper code formatting is always a problem and it's good to automate this, but I don't want to be fighting with a system that is "fixing" deliberate side-steps from the established convention, like in the examples you show.

[Arpafaucon]

I use VSCode; I don't know how much this applies to other users.

Personnally (and in my company), I hate the "format on save" option of VSCode, because it's acting behind your back.

• In python source code, we adopted black with its strict & deterministic strategy. I like not having to discuss style issues in PR anymore. If you need to deviate from the formatting (typically when entering big arrays of numbers in a matrix-like format) you can always insert comments to locally disable the reformatting. We enforce this in CI, works well in practice, I think
• for C++, we haven't yet found a default good enough everywhere. So I encourage to use the "format selection" feature to only reformat the lines thet were just edited. This way, diffs stay small, but we get a tool to fix all the minor style issues and have a overall coherent style. We don't enforce this in CI, and assume devs are mature enough (not always the case, though, but we manage). That's what I would suggest for this repo, at least to start

[wolfpld]

I have cherry-picked the commit. Formatting is done only on demand, so it's fine.