[smart_holder] .def_readonly, .def_readwrite adaptors (continuation of PR #3581). (#3844)

* Transferred net diff from PR #3581, as-is.

* Automatic `pre-commit run --all-files` fixes. NO manual changes.

* Removing trailing `//` (originally added to manipulate clang-format), as suggested by @charlesbeattie back in Jan/Feb under PR #3581.

* Renaming `xetter_cpp_function` to `property_cpp_function` as suggested by @rainwoodman

* Fully explain the terse variable naming scheme in test_class_sh_property (as suggested by @rainwoodman)

* Also use parametrize for readonly, readwrite (as suggested by @rainwoodman)

* Apply change suggested by @skylion007 (with clang-format).