[libc][wchar] implement wcslen (#124150)

Update string_utils' string_length to work with char* or wchar_t*, so that it
may be reusable when implementing wmemchr, wcspbrk, wcsrchr, wcsstr.

Link: #12

[libc][wchar] implement wcslen (#124150)

Update string_utils' string_length to work with char* or wchar_t*, so that it
may be reusable when implementing wmemchr, wcspbrk, wcsrchr, wcsstr.

Link: #121183
Link: #124027

Co-authored-by: Nick Desaulniers <ndesaulniers@google.com>

---------

Co-authored-by: Tristan Ross <tristan.ross@midstall.com>

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