LibTextCodec: Add ISO-8859-15 (aka Latin-9) encoding

Author: https://github.com/aatosmajava 🔰
Commit: 7597cca5c6
Pull-request: https://github.com/SerenityOS/serenity/pull/8063
Reviewed-by: https://github.com/IdanHo ✅

Userland/Libraries/LibTextCodec/Decoder.h

@@ -48,6 +48,11 @@ public:
     virtual String to_utf8(const StringView&) override;
 };
 
+class Latin9Decoder final : public Decoder {
+public:
+    virtual String to_utf8(const StringView&) override;
+};
+
 Decoder* decoder_for(const String& encoding);
 Optional<String> get_standardized_encoding(const String& encoding);
 bool is_standardized_encoding(const String& encoding);