Charset: Introduce WP_Encoding for standardized handling.

[dmsnell]

Trac ticket: Core-64473

Prep work for Core-64427

Introduces a class to contain relevant WHATWG spec-compliant handling of character encodings, conversions, and recognition.

Answers two valuable questions:

• Given this charset description, what charset is it?
• What charsets should WordPress support?

Later on, this will:

• Provide fallback decoders/encoders for supported types.
• Infer charset from a byte stream.