[clang-format] Add `extern` and `register` options for `QualifierOrder` setting

Currently [`QualifierOrder`](https://clang.llvm.org/docs/ClangFormatStyleOptions.html#qualifierorder) supports `const`, `inline`, `static`, `friend`, `constexpr`, `volatile`, `restrict`, `type`.

I think it would be appropriate to add the `extern` and `register` qualifier to the list of options.

Start with the clang-format implementation and documentation for the QualifierOrder setting, using the linked QualifierOrder option as the entry point. Trace the existing qualifier options and their tests, then add extern and register consistently and verify that the setting accepts and orders them as documented.

Written by the indexing model from the issue text.