Define a clang-format file about managarm

[Dennisbonke]

It's in the works, I expect one in the coming weeks. I have it ready but it's blocked on llvm 16, which I'll PR in tomorrow hopefully.

[ArsenArsen]

https://github.com/ArsenArsen/managarm/blob/clang-reformat/.clang-format

[Dennisbonke]

I did some testing and some small tweaks. Also enabled some options that are only in llvm 15 or above, hence the llvm upgrade

[Dennisbonke]

See #528

[cflaviu]

Most Linux distributions do not provide the latest clang-format version, v16.
It would be more useful to target version v14 or v15 to cover a broader range of users. In my case I have by default v14 but I can install v15, but not v16.
clang-format doesn't offer enough configuration granularity to describe a specific coding style and it is expected to make some compromises regarding the formatting, eg. in my particular case, the initialization lists. IMO it's not worth spending too much time finding the perfect tool or the perfect configuration to match the expected coding style.

[ArsenArsen]

you can use host-llvm-toolchain to run clang-format