path: remove `StringPrototypeCharCodeAt` from `posix.extname`

Benchmark shows an improvement:

                                                                                confidence improvement accuracy (*)   (**)   (***)
path/extname-posix.js n=100000 path=''                                                  *      5.10 %       ±4.68% ±6.23%  ±8.14%
path/extname-posix.js n=100000 path='/'                                                 *      4.92 %       ±3.78% ±5.04%  ±6.55%
path/extname-posix.js n=100000 path='/foo'                                              *      6.57 %       ±6.24% ±8.32% ±10.88%
path/extname-posix.js n=100000 path='/foo/bar/baz/asdf/quux.foobarbazasdfquux'        ***      7.94 %       ±2.34% ±3.12%  ±4.08%
path/extname-posix.js n=100000 path='/foo/bar/baz/asdf/quux'                            *      5.77 %       ±4.68% ±6.24%  ±8.15%
path/extname-posix.js n=100000 path='foo/.bar.baz'                                      *      4.49 %       ±4.40% ±5.88%  ±7.69%
path/extname-posix.js n=100000 path='foo/bar/...baz.quux'                             ***     11.69 %       ±6.29% ±8.37% ±10.90%
path/extname-posix.js n=100000 path='foo/bar/..baz.quux'                               **      7.95 %       ±5.30% ±7.05%  ±9.17%
path/extname-posix.js n=100000 path='index.html'                                        *      6.52 %       ±5.33% ±7.10%  ±9.28%
path/extname-posix.js n=100000 path='index'                                                    3.09 %       ±3.90% ±5.19%  ±6.78%

Be aware that when doing many comparisons the risk of a false-positive
result increases. In this case, there are 10 comparisons, you can thus
expect the following amount of false-positive results:
  0.50 false positives, when considering a   5% risk acceptance (*, **, ***),
  0.10 false positives, when considering a   1% risk acceptance (**, ***),
  0.01 false positives, when considering a 0.1% risk acceptance (***)