[String] New locale aware casing methods

[bram123]

Q	A
Branch?	7.1
Bug fix?	no
New feature?	yes
Deprecations?	no
Tickets	Fix #52161
License	MIT

Adds new localeUpper/localeLower/localeTitle methods to the AbstractUnicodeString class

• To change the string case according to locale-specific case mappings

Code examples:

$string = new UnicodeString('άδικος');
echo $string->upper(), PHP_EOL; // ΆΔΙΚΟΣ
echo $string->localeUpper('el'), PHP_EOL; // ΑΔΙΚΟΣ

$string = new UnicodeString('ijssel');
echo $string->title(), PHP_EOL; // Ijssel
echo $string->localeTitle('nl'), PHP_EOL; // IJssel

$string = new UnicodeString('İSTANBUL');
echo $string->lower(), PHP_EOL; // i̇stanbul (LATIN SMALL LETTER I COMBINING DOT ABOVE)
echo $string->localeLower('tr'), PHP_EOL; // istanbul (LATIN SMALL LETTER I)

[nicolas-grekas]

Can you please also add a line to the changelog of the component?

[nicolas-grekas]

Two minor nitpicking and GTM, thanks.

[smnandre]

Re-copying this there in case we need it in a following PR or to write doc :

--> official ICU list of special casings

[fabpot]

Thank you @bram123.