Add Cloudflare Web Analytics as a tracker

[dotZak]

Cloudflare has recently released their web analytics platform and it is available for integration.

Cloudflare Web Analytics is marketed as, "Privacy-first, lightweight, accurate web analytics—for free".

This PR includes the basic script tag allowing users to install the analytics script more simply if they're already using jekyll-analytics.

This PR does not include the more advanced functionality (disabling SPA tracking, or GTM integration) as those require a more advanced configuration with some additional conditionals. If they're deemed desirable they can be added, but I think someone more familiar with the jekyll-analytics gem would need to consult on them.