LibTextCodec+LibWeb: Move isomorphic coders to LibTextCodec

This will be used outside of LibWeb.

Libraries/LibTextCodec/Decoder.cpp

@@ -1299,4 +1299,18 @@ ErrorOr<void> ReplacementDecoder::process(StringView input, Function<ErrorOr<voi
     return {};
 }
 
+// https://infra.spec.whatwg.org/#isomorphic-decode
+String isomorphic_decode(StringView input)
+{
+    // To isomorphic decode a byte sequence input, return a string whose code point length is equal to input’s length
+    // and whose code points have the same values as the values of input’s bytes, in the same order.
+    // NB: This is essentially spec-speak for "Decode as ISO-8859-1 / Latin-1".
+    StringBuilder builder(input.length());
+
+    for (auto byte : input.bytes())
+        builder.append_code_point(byte);
+
+    return builder.to_string_without_validation();
+}
+
 }

Libraries/LibTextCodec/Decoder.h

@@ -135,4 +135,6 @@ TEXTCODEC_API ErrorOr<String> convert_input_to_utf8_using_given_decoder_unless_t
 
 TEXTCODEC_API StringView get_output_encoding(StringView encoding);
 
+TEXTCODEC_API String isomorphic_decode(StringView);
+
 }

Libraries/LibTextCodec/Encoder.cpp

@@ -667,4 +667,23 @@ ErrorOr<void> SingleByteEncoder<ArrayType>::process(Utf8View input, Function<Err
     return {};
 }
 
+// https://infra.spec.whatwg.org/#isomorphic-encode
+ByteString isomorphic_encode(StringView input)
+{
+    // To isomorphic encode an isomorphic string input: return a byte sequence whose length is equal to input’s code
+    // point length and whose bytes have the same values as the values of input’s code points, in the same order.
+    // NB: This is essentially spec-speak for "Encode as ISO-8859-1 / Latin-1".
+    StringBuilder builder(input.length());
+
+    for (auto code_point : Utf8View { input }) {
+        // VERIFY(code_point <= 0xFF);
+        if (code_point > 0xFF)
+            dbgln("FIXME: Trying to isomorphic encode a string with code points > U+00FF.");
+
+        builder.append(static_cast<u8>(code_point));
+    }
+
+    return builder.to_byte_string();
+}
+
 }

Libraries/LibTextCodec/Encoder.h

@@ -90,4 +90,6 @@ private:
 TEXTCODEC_API Optional<Encoder&> encoder_for_exact_name(StringView encoding);
 TEXTCODEC_API Optional<Encoder&> encoder_for(StringView label);
 
+TEXTCODEC_API ByteString isomorphic_encode(StringView);
+
 }