Bugfixes 🐛

• Ignore NeverType in subtractable types (#389), thanks @jlherren!
• Try fixing CpuCoreCounter problem (phpstan/phpstan-src@0e32301), #4147
• is_a(): specify class-string types if allow_string: true (#392), #4124, thanks @jiripudil!
• CallableType: accept [class-string, constant-string] array as callable (#394), #2880, thanks @jiripudil!
• is_a(): support class-string in parameter $class (#393), #2799, thanks @jiripudil!
• Support "null" and "false" pseudotypes in native unions (#400), #4198, #4179, thanks @jiripudil!

Function signature fixes 🤖

• Fix a misplaced null in XMLWriter::startElementNS (#388), thanks @spaze!
• sync some more types with PhpStorm stubs (#387), thanks @voku!
• Fix ftp_mlsd (#399), thanks @enumag!