Charset: Polyfill mb_ord() and mb_chr().

[dmsnell]

Trac ticket: Core-65342

See also Core-62172
Preparation for #11567

These functions are useful primitives but missing when the mbstring extension isn’t available. This patch adds polyfills for those few environments where this is the case so that WordPress code can unconditionally call them.

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[github-actions]

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