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.


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