Add .clang-format to have consistent formatting.

[ipochto]

Maybe increase the column limit?

[Jarod42]

It would just be me, it would be even less (80)

Whereas 80x24 "char screen" is no longer the norm :-) we might easily

• split screen,
• use tool (as diff/merge), which often show code side by side. (so up to 3*100 with merge 3-way tools)

It is not in the CI, so it is mostly currently just a guideline.