Introduce current_datetime() for better time operations

[Rarst]

WordPress uses current_time() function to refer to the current moment of date and time. Timestamps issues in it aside, it's not friendly to time operations, leading to convoluted and fragile code when time needs to be shifted from current, for example:

<?php
$tomorrow = gmdate( 'Y-m-d', strtotime( '+1 day', current_time( 'timestamp' ) ) );

The patch introduces current_datetime() analogue that returns proper DateTimeImmutable representation of current moment in time, open to a more flexible and reliable use:

<?php
tomorrow = current_datetime()->modify('+1 day')->format('Y-m-d');

Patch needs wp_timezone() merged, see #24730

Parent issue #40657

[SergeyBiryukov]

In 45883:

Date/Time: Introduce current_datetime() for better time operations.

Returning a DateTimeImmutable representation of the current moment in time, this allows for a more flexible and reliable use than current_time() provides.

Props Rarst.
Fixes #47464.