LibTextCodec+AK: Don't validate UTF-8 strings twice

UTF8Decoder was already converting invalid data into replacement
characters while converting, so we know for sure we have valid UTF-8
by the time conversion is finished.

This patch adds a new StringBuilder::to_string_without_validation()
and uses it to make UTF8Decoder avoid half the work it was doing.

AK/StringBuilder.cpp

@@ -156,6 +156,11 @@ ErrorOr<String> StringBuilder::to_string() const
     return String::from_utf8(string_view());
 }
 
+String StringBuilder::to_string_without_validation() const
+{
+    return String::from_utf8_without_validation(string_view().bytes());
+}
+
 ErrorOr<FlyString> StringBuilder::to_fly_string() const
 {
     return FlyString::from_utf8(string_view());