LibLine: Support wrapping lines in actual_rendered_string_metrics

This will allow us to use this API to split an input to visible lines of
a specified width.

Userland/Libraries/LibLine/StringMetrics.h

@@ -20,6 +20,8 @@ struct StringMetrics {
     struct LineMetrics {
         Vector<MaskedChar> masked_chars;
         size_t length { 0 };
+        size_t visible_length { 0 };
+        Optional<size_t> bit_length { 0 };
 
         size_t total_length() const { return length; }
     };