clang-format: Treat "." in protos like namespace separators.

Before:
  optional really.really.long.and.qualified.type.aaaaaaa
      .aaaaaaaa another_fiiiiiiiiiiiiiiiiiiiiield = 2;

After:
  optional
     really.really.long.and.qualified.type.aaaaaaa.aaaaaaaa
          another_fiiiiiiiiiiiiiiiiiiiiield = 2;