MB_CASE_FOLD doesn't handle ẞ character properly

[colinodell]

Summary

It seems that this polyfill incorrectly case-folds the ẞ character to ß instead of ss.

Details

Running this code on PHP 7.3 with mbstring installed:

echo mb_convert_case('ẞ', MB_CASE_FOLD);

Will produce this output: ss

However, when I run that same input through this polyfill (master branch):

echo \Symfony\Polyfill\Mbstring\Mbstring::mb_convert_case('ẞ', \Symfony\Polyfill\Mbstring\Mbstring::MB_CASE_FOLD);

I instead get ß. This does not match mbstring's behavior on PHP 7.3.

[colinodell]

It looks like this occurs with both U+1E9E LATIN CAPITAL LETTER SHARP S and U+00DF LATIN SMALL LETTER SHARP S. Both of these should return ss when case-folded.

[nicolas-grekas]

PHP 7.3 uses full case folding.
This is implemented in tchwork/utf8, so we could borrow the map from there:
https://github.com/tchwork/utf8/blob/30ec6451aec7d2536f0af8fe535f70c764f2c47a/src/Patchwork/Utf8.php#L168-L170

Up for a PR?