Add conditional return type for sanitize_post_field

[IanDelMar]

The return type of sanitize_post_field() depends on both the $field being sanitised and the $context.

For integer fields

$int_fields = array( 'ID', 'post_parent', 'menu_order' );

it returns an integer. If $value is of type int, we can even return a constant integer or an integer range.

For the field value 'ancestors, it returns an array with non-negative integer values.

For all other fields, the type depends on the $context:

• 'raw': $value is returned as is, which is handled via the template T; for constant types, the same constant type is returned.
• 'attribute'|'edit'|'js': $value is escaped--and may be transformed--using esc_attr(), esc_html(), or esc_js(). Assuming the filters in these functions do not alter the type, the returned type is string, even if $value is a constant string. The only exception is the field value 'post_content' which might be filtered but is only escaped if user_can_richedit() === false. In this case the default return type of mixed is retained.
• All other $context values: $value is not escaped but is filtered, with the filter callback documented as returning mixed. Therefore, the default return type of mixed is retained.

Related: #382, szepeviktor/phpstan-wordpress#297