Add UTF-8 decoder

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Transcode from UTF-8 to X

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Transcoding from UTF-8 to X requires knowledge how X is structured (encoded). As such, I don't think this is something that can be solved for the general case. As an alternative #16 and #17 (and others) suggest adding dedicated decoders (and encoders) for Latin1 (ISO 8859-1), UTF-16 and other common encodings. We may add more encodings in the future and/or add a way to interface with iconv or PHP's multibyte functions, but other than that I have no use case for this 👍

Arguably, transcoding from UTF-8 to X is somewhat easier than the other way around (transcoding from X to UTF-8 as discussed in #4) because we can rely on UTF-8 input encoding and only need an interface for converting to the target encoding.