[clang-format] add support for cppm files

C++20 Modules current style is to assign a new file suffix .cppm.

https://github.com/llvm/llvm-project/issues/52658

Ensure git-clang-format can handle that as a default extension

Reviewed By: ChuanqiXu, HazardyKnusperkeks, curdeius

Differential Revision: https://reviews.llvm.org/D115625

Fixes: #52658