Function normalizeEncoding

Convert an encoding label to its name, as an ASCII-lowercased string.

If an encoding with that label does not exist, returns null.

This is the same as decoder.encoding getter, except that it:

1. Supports replacement encoding and its labels
2. Does not throw for invalid labels and instead returns null

It is identical to:

labelToName(label)?.toLowerCase() ?? null
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All encoding names are also valid labels for corresponding encodings.