Classic Theme Helper Plugin: Ensure Featured Content and imported / required files in the package are initialized

[coder-karen]

Fixes https://github.com/Automattic/vulcan/issues/384

Proposed changes:

• This PR ensures the classes that initialize both Featured Content and Responsive Videos from the Classic Theme Helper package are initialized in the plugin, so they can work even if Jetpack is not also initializing those classes.

Other information:

• [ ] Have you written new tests for your changes, if applicable?
• [ ] Have you checked the E2E test CI results, and verified that your changes do not break them?
• [ ] Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

https://github.com/Automattic/vulcan/issues/384

Does this pull request change what data or activity we track or use?

No.

Testing instructions:

To replicate the issue with this plugin not initializing Responsive Videos and Featured Content:

• Testing locally, make sure you have installed and built the Classic Helper Theme plugin: jetpack install plugins/classic-theme-helper-plugin, jetpack build plugins/classic-theme-helper-plugin, then activate the plugin.
• Comment out responsive-videos and featured-content from projects/plugins/jetpack/modules/module-extras.php Rebuild the Jetpack plugin - jetpack build plugins/jetpack)
• Follow the steps in this PR to test Responsive Videos and Featured Content (only on self-hosted): https://github.com/Automattic/jetpack/pull/37969
• They shouldn't work.

To test the fix:

• Apply this PR and follow the same steps as above.
• Responsive Videos and Featured Content should work as expected.