Optimize repo clone

[erseco]

This pull request introduces optimizations to the editor source fetching process in the Makefile, aiming to reduce unnecessary downloads and improve build efficiency. The changes add sparse and shallow cloning, reuse existing checkouts when possible, and provide clearer cleaning targets for the build artifacts and fetched sources.

Optimizations to editor source fetching:

• Added sparse-checkout and --filter=blob:none to the fetch-editor-source target, significantly reducing download size by excluding unnecessary directories (test/, doc/, app/, packaging/).
• Implemented a .fetched-ref marker and logic to reuse existing checkouts if the repo URL and ref haven't changed, avoiding redundant fetches.

Improved build cleanup:

• Enhanced clean-editor to remove the .fetched-ref marker and clarified its scope. Added a new clean-editor-all target to remove the fetched source entirely.
• Updated the help output to reflect the new and modified cleaning targets, providing clearer guidance to users.

[github-actions]

Test in WordPress Playground

Test the plugin with the code from this branch:

Open in WordPress Playground

⚠️ The embedded eXeLearning editor is not available in this preview (requires build). All other plugin features (ELP upload, shortcode, Gutenberg block, preview) work normally.