Introduce is_private() conditional

[sillybean]

Add a quick conditional tag to check whether the post is private.

[johnbillion]

Thanks for the patch, @sillybean. Some feedback for you:

• This probably needs a more descriptive function name, such as is_private_post().
• Have you tested this function? It's checking the value of the post_password field.
• This will need unit tests to avoid the above.

[donmhico]

Added new function is_private_post() and unit test for it.

[donmhico]

My attached patch, 37462.diff, introduced a new function - is_private_post( $post ) - which returns true if the $post passed is private and false if otherwise. It uses the function get_post_status().

Reference:
​https://developer.wordpress.org/reference/functions/get_post_status/

[donmhico]

Correct @since to adhere to WPCS.