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.cpp

@@ -12,6 +12,7 @@
 #include <AK/String.h>
 #include <AK/StringBuilder.h>
 #include <AK/StringView.h>
+#include <AK/Utf16View.h>
 #include <AK/Utf32View.h>
 
 namespace AK {
@@ -112,6 +113,16 @@ void StringBuilder::append_code_point(u32 code_point)
     }
 }
 
+void StringBuilder::append(Utf16View const& utf16_view)
+{
+    for (size_t i = 0; i < utf16_view.length_in_code_units();) {
+        auto code_point = utf16_view.code_point_at(i);
+        append_code_point(code_point);
+
+        i += (code_point > 0xffff ? 2 : 1);
+    }
+}
+
 void StringBuilder::append(Utf32View const& utf32_view)
 {
     for (size_t i = 0; i < utf32_view.length(); ++i) {

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);

Kernel/CMakeLists.txt

@@ -312,6 +312,8 @@ set(AK_SOURCES
     ../AK/Time.cpp
     ../AK/Format.cpp
     ../AK/UUID.cpp
+    ../AK/Utf8View.cpp
+    ../AK/Utf16View.cpp
 )
 
 set(ELF_SOURCES