Rename date.php to class-wp-date-query.php

[SergeyBiryukov]

Background: #47632

[45663] renamed wp-includes/class-oembed.php to wp-includes/class-wp-oembed.php to conform to ​naming conventions in WordPress coding standards.

The same should be done to the WP_Date_Query class, which currently resides in wp-includes/date.php and should be moved to wp-includes/class-wp-date-query.php.

[SergeyBiryukov]

In 45678:

Coding Standards: Move wp-includes/date.php to wp-includes/class-wp-date-query.php.

This renames the file containing the WP_Date_Query class to conform to the coding standards.

This commit also includes:

• A new date.php that includes the new file, for anyone that may've been including the file directly.
• Replaces references to the old filename with the new filename.

Fixes #47775. See #47632.

[SergeyBiryukov]

In 45679:

Coding Standards: Fix newlines in wp-includes/date.php.

See #47775.