Charset: Introduce wp_utf8_chunks() to iterate through strings.

[dmsnell]

Trac ticket: Core-63863
See: #9825, #9830, #9498, (#9826), #9827, #9798, #9828, #9829

Introduces a new method, wp_utf8_chunks(), for iterating through a string which might contain invalid spans of UTF-8 bytes.

This function is largely a low-level function for working with text data. With it, it’s possible to operate on the valid UTF-8 portions of a string while preserving the invalid parts. For instance, this might be used to operate on improperly-constructed JSON data.

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[dmsnell]

Without a clear and compelling need to abstract this in Core I’m closing for now. It’s easy enough to create in an application-specific way. If such a need emerges we can re-open this for later consideration.