Add explicit IgnoreBOM and ErrorMode options to LibTextCodec decoders,
and thread them through TextDecoder and TextDecoderStream.
This lets Web-facing decoder APIs preserve BOMs when requested and use
fatal error handling without post-processing decoded output.
NB: RemoveBOM was renamed to IgnoreBOM as "RemoveBOM" is the name
used by encoding_rs and was previously an implementation detail.
The new name matches what is used by the encoding standard as it
is now also used in LibWeb.
