Add .clang-format and corresponding CI pass to check code format

Like [llvm-project](https://github.com/llvm/llvm-project), I think we can add a `.clang-format` file in the root dir (and the file content is pretty simple since we just use the LLVM-style code format `BasedOnStyle: LLVM`, refer to https://github.com/llvm/llvm-project/blob/main/.clang-format), and then add a github action to check every PR commit to ensure the code style is aligned before it is merged.