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): https://github.com/nginx/unit/pull/1471
• Chromium: https://github.com/nginx/unit/pull/1470
• Microsoft: https://github.com/nginx/unit/pull/1472
• LLVM: https://github.com/nginx/unit/pull/1473
• GNU: https://github.com/nginx/unit/pull/1474