[New feature] Add check style in code for only changed/added files in PR

[den-patrakeev]

Hi!

Add check style in code for only changed/added files in PR.

Since I did not find a code style agreement in the project, I prepared a minimal configuration based on LLVM coding standards and force standart C++11.

Official docs used for config: https://clang.llvm.org/docs/ClangFormatStyleOptions.html

I propose to implement this to check modified and new C++ files without affecting the existing code base.

Example of check triggering: https://github.com/den-patrakeev/coolreader/actions/runs/12502186857/job/34880772968?pr=1