Add .clang-format file with Unit ruleset

[javorszky]

This adds a .clang-format file that can be used with the clang-format tool to either reformat code locally, or automatically flag up code styling issues during a github action.

This PR's only aim right now is to have a conversation about the rulesets and come to a standard (ie ruleset in the clang-format file) that the entire team agrees on, so we can get to a point where we're not having conversations about code styling in future PRs.

This PR does not aim to wire it up to github actions yet.

Diffs of the codebase:

• custom style (this .clang-format): Clang format custom style #1471
• Chromium: Clang format Chromium style #1470
• Microsoft: Gabor/clang format microsoft #1472
• LLVM: Clang format LLVM #1473
• GNU: Clang format GNU #1474

[callahad]

@javorszky What's the current diff if you apply these rules to Unit?

[ac000]

How was this file generated and where do all the comments come from?

[javorszky]

How was this file generated and where do all the comments come from?

Originally with clang-format --dump-config --style=llvm > .clang-format, and then I went over every single rule there, looked up what they do on the documentation site, compared existing codebase to the values where I needed to make a decision.

Every comment is written by hand by me.

[ac000]

Hmm

$ clang-format --dry-run src/nxt_php_sapi.c 
/home/andrew/src/unit/.clang-format:51:3: error: unknown key 'AlignFunctionDeclarations'
  AlignFunctionDeclarations: true
  ^~~~~~~~~~~~~~~~~~~~~~~~~
Error reading /home/andrew/src/unit/.clang-format: Invalid argument

[javorszky]

need clang-format v20.0.0 sadly

[ac000]

That's way too new!, it's not even released yet, probably in March next year...

[javorszky]

I'll put up a new version with the v20 rules removed, and a diff for this, and all the other prebuilt styles as well

[javorszky]

This has been updated so clang-format 19 will take it. I've also updated the main comment to point to a bunch of different draft PRs for the reformats. (if you don't see it, I'm still pushing / opening them)