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.h

@@ -73,6 +73,7 @@ public:
     [[nodiscard]] ByteString to_byte_string() const;
 #endif
 
+    [[nodiscard]] String to_string_without_validation() const;
     ErrorOr<String> to_string() const;
     ErrorOr<FlyString> to_fly_string() const;