Add return type extensions for sanitize_post

[IanDelMar]

See php-stubs/wordpress-stubs#381

[szepeviktor]

@IanDelMar Will it be better than php-stubs/wordpress-stubs#37 ?

[IanDelMar]

It will be correct. Using generics gives correct types only if the type is not a constant type or the function has no effect. For example, before php-stubs/wordpress-stubs#369 and #296, wp_slash("o'reilly") gave "o'reilly", while it now gives 'o\'reilly'. Before php-stubs/wordpress-stubs#381, post_sanitize(['ID' => 'string']) gave ['ID' => 'string']; currently it gives array. With a return type extension, it would give ['ID' => 0] - just like a call to post_sanitize would.

The problem with generics is described in this issue: php-stubs/wordpress-stubs#333