Port mbstring to use EXTENSIONS

Consolidate the usage of "either" and "one of" in error messages

Closes GH-6173

Promote invalid case mode to ValueError in mb_case_converter

Add assertions to check the return value is not NULL as this indicates a bug.
Add identical assertion to mb_strtoupper and mb

Promote invalid case mode to ValueError in mb_case_converter

Add assertions to check the return value is not NULL as this indicates a bug.
Add identical assertion to mb_strtoupper and mb_strtolower.
This means these functions can't return false anymore, ammend stubs accordingly.

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