AK+Kernel: Add StringBuilder::append overload for UTF-16 views

Currently, to append a UTF-16 view to a StringBuilder, callers must
first convert the view to UTF-8 and then append the copy. Add a UTF-16
overload so callers do not need to hold an entire copy in memory.

AK/StringBuilder.h

@@ -22,6 +22,7 @@ public:
     ~StringBuilder() = default;
 
     void append(StringView const&);
+    void append(Utf16View const&);
     void append(Utf32View const&);
     void append(char);
     void append_code_point(u32);