Move WP_Object_Cache into its own file

[SergeyBiryukov]

Background: #33413, #37035, #37209, #37827, #47775.

In the tickets linked above, most of WP core classes were moved to their own files to conform to ​naming conventions in WordPress coding standards.

The same should be done for the WP_Object_Cache class, which currently resides in wp-includes/cache.php and should be moved to wp-includes/class-wp-object-cache.php.

For backward compatibility, the new file should be included from wp-includes/cache.php.

[SergeyBiryukov]

In 47197:

Coding Standards: Move WP_Object_Cache to wp-includes/class-wp-object-cache.php.

This ensures the file containing the WP_Object_Cache class conforms to the coding standards.

For backward compatibility, the new file is included from wp-includes/cache.php.

Fixes #49373. See #49222.

[SergeyBiryukov]

@xkon Sorry, should have commented that I already had the changes locally and was just waiting for the tests to pass :)

[xkon]

Haha no problem at all @SergeyBiryukov ! I was pretty sure that you might've been cooking a patch as well. I didn't refresh the page here either before uploading it so I missed the commit comment 😂 .

All good!