AK: Add a StringView method to count the number of lines in a string

We already have a helper to split a StringView by line while considering
"\n", "\r", and "\r\n". Add an analagous method to just count the number
of lines in the same manner.

AK/StringView.h

@@ -240,6 +240,7 @@ public:
         Yes,
     };
     [[nodiscard]] Vector<StringView> lines(ConsiderCarriageReturn = ConsiderCarriageReturn::Yes) const;
+    [[nodiscard]] size_t count_lines(ConsiderCarriageReturn = ConsiderCarriageReturn::Yes) const;
 
     // Create a new substring view of this string view, starting either at the beginning of
     // the given substring view, or after its end, and continuing until the end of this string