Add prism-js to the classes template

[tomalec]

Changes proposed in this Pull Request:

Add prism-js to the classes template to enable code highlighting for code samples in the PHPDocs class.

How to test the changes in this Pull Request:

1. Generate code reference for the latest release: ./deploy.sh -v --build-only -s 9.1.4
2. Edit some file locally to add code snippet inside doc block.:
   ./woocommerce/src/Admin/API/Reports/DataStore.php

/**
 * Class description
 *
 * Example:
 * <pre><code class="language-php">class MyDataStore extends DataStore implements DataStoreInterface {
 *     /** Cache identifier. &ast;/
 *     protected $cache_key = 'my_example';
 * </code></pre>
 *
 */
class DataStore extends SqlQuery implements DataStoreInterface {

5. Generate again from local changes ./deploy.sh -v --build-only -s 9.1.4 --no-download
6. open file:///path/to/code-reference/build/api/classes/Automattic-WooCommerce-Admin-API-Reports-DataStore.html

Before

After

[eason9487]

Thanks for the improvement! Tested with newly merged changes in WooCommerce core and it works well. LGTM.