Coding Standards: Move wp-includes/class-oembed.php to `wp-includes…

…/class-wp-oembed.php`.

This renames the file containing the `WP_oEmbed` class to conform to the coding standards.

This commit also includes:

- A new `class-oembed.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.

See #47632.



git-svn-id: https://develop.svn.wordpress.org/trunk@45663 602fd350-edb4-49c9-b593-d223f7449a82