AK: Add String{View,}::find(StringView)

I personally mistook `find_first_of(StringView)` to be analogous to this
so let's add a `find()` method that actually searches the string.

AK/StringUtils.cpp

@@ -25,6 +25,7 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#include <AK/MemMem.h>
 #include <AK/Memory.h>
 #include <AK/Optional.h>
 #include <AK/String.h>
@@ -342,6 +343,13 @@ StringView trim_whitespace(const StringView& str, TrimMode mode)
 
     return str.substring_view(substring_start, substring_length);
 }
+
+Optional<size_t> find(const StringView& haystack, const StringView& needle)
+{
+    return AK::memmem_optional(
+        haystack.characters_without_null_termination(), haystack.length(),
+        needle.characters_without_null_termination(), needle.length());
+}
 }
 
 }