add clang-format

This introduces clang-format to enforce a consistent code style for Stockfish.

Having a documented and consistent style across the code will make contributing easier
for new developers, and will make larger changes to the codebase easier to make.

To facilitate formatting, this PR includes a Makefile target (`make format`) to format the code,
this requires clang-format (version 17 currently) to be installed locally.

Installing clang-format is straightforward on most OS and distros
(e.g. with https://apt.llvm.org/, brew install clang-format, etc), as this is part of quite commonly
used suite of tools and compilers (llvm / clang).

Additionally, a CI action is present that will verify if the code requires formatting,
and comment on the PR as needed. Initially, correct formatting is not required, it will be
done by maintainers as part of the merge or in later commits, but obviously this is encouraged.

fixes https://github.com/official-stockfish/Stockfish/issues/3608
closes https://github.com/official-stockfish/Stockfish/pull/4790

Co-Authored-By: Joost VandeVondele <Joost.VandeVondele@gmail.com>

CONTRIBUTING.md

@@ -57,8 +57,9 @@ discussion._
 
 ## Code Style
 
-We do not have a strict code style. But it is best to stick to the existing
-style of the file you are editing.
+Changes to Stockfish C++ code should respect our coding style defined by
+[.clang-format](.clang-format). You can format your changes by running
+`make format`. This requires clang-format version 17 to be installed on your system.
 
 ## Community and Communication