feat: add Super Page Cache option to the onboarding wizard

[HardeepAsrani]

Summary

This adds a separate Super Page Cache upsell to the LightStart onboarding wizard without changing the existing Optimole option.

Will affect visual aspect of the product

YES/NO

Screenshots

Test instructions

Fresh install state

• Open the onboarding wizard with Super Page Cache not installed.
• Confirm the new Super Page Cache option is visible and checked by default.
• Choose a template and continue.
• Verify Super Page Cache is installed and activated.

Installed but inactive state

• Install Super Page Cache manually but leave it inactive.
• Open the onboarding wizard.
• Confirm the Super Page Cache option is still visible.
• Choose a template and continue.
• Verify the plugin is activated without attempting a fresh install.

Already active state

• Activate Super Page Cache before opening the wizard.
• Open the onboarding wizard.
• Confirm the Super Page Cache option is not shown.

Optimole regression check

• Repeat the same basic onboarding flow with Optimole not installed.
• Confirm the existing Optimole option still appears and still installs/activates correctly.

Combined flow check

• With both Optimole and Super Page Cache inactive or missing, run the wizard with both options checked.
• Verify both plugins complete successfully and the template import still finishes as expected.

Skip behavior

• Leave the WPSC checkbox unchecked and continue through the wizard.
• Verify Super Page Cache is not installed or activated.

Check before Pull Request is ready:

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

Closes https://github.com/Codeinwp/WP-Cloudflare-Super-Page-Cache/issues/311.

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🎉 This PR is included in version 2.6.22 🎉

The release is available on GitHub release

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