Add encapsulated-plugin compatibility for Zen Cart 2.2.1

[Iscarly-Mr]

This PR adds compatibility for using SitemapXML as an encapsulated plugin on Zen Cart 2.2.1 while retaining support for the classic installation layout.

Summary of changes:

• In storefront header_php.php, load sitemapxml.php from the plugin-local classes directory when present, falling back to the classic core path otherwise.
• In storefront header_php.php, discover sitemapxml_*.php generator modules from the plugin-local page directory first, then fall back to the classic core path.
• In YOUR_Admin/sitemapxml.php, discover the available sitemap generator modules from the plugin-local directory first, then fall back to the classic core path.
• Update the README to note Zen Cart 2.2.1 support and mention encapsulated-plugin usage and file-permission considerations.

Why:

• When packaged as an encapsulated plugin in zc_plugins, the current v4.0.5 code still looks for the sitemap class and generator modules in the classic core locations. That leaves the admin generator list empty and prevents sitemap generation.
• These changes allow the same codebase to work in both classic and encapsulated installation styles.

Validation:

• Tested with Zen Cart 2.2.1 in an encapsulated-plugin deployment.
• Verified that the admin page lists the sitemap generator modules correctly.
• Verified that sitemap generation works after installation and configuration.
• Ran PHP lint on the modified files.

[lat9]

I'm sorry, but while I applaud your efforts I can't accept this change as it's not a "proper" encapsulated plugin. When the SitemapXML "goes encapsulated", it will drop support for the unencapsulated/classic versions and make modifications to its distribution to host all files in their zc_plugins directory. It'll also remove use of the admin's init_includes files, moving those to a ScriptedInstaller.