Encoding: Introduce WHATWG-compliant encoding name canonicalization.

[dmsnell]

Trac ticket: Core-66259

See #10640 (introduce MIME content type inference).

Introduces a character encoding name canonicalization, coupled with a decoder and encoder interface that code can call to reliably interact with web-standards byte streams and a provided encoding name.

Status

• Needs tests that don’t simply duplicate the existing logic.

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