Multiplexed Google Analytics support GA4 with UA backward compatibility

[sohale]

Dual Compatibility for Google Analytics Support: v4 (GA4) and UA

This PR provides a sustainable solution for issue #63 by enabling the theme to support both Google Analytics v4 (GA4) and Universal Analytics (UA) tracking IDs.

The user only sets the google_analytics_v4:. The legacy users can keep using google_analytics:. The user does not even need to be aware of the other version. It feels like auto-detection.

Key Features

• Backward compatibility: Existing sites using only UA or only GA4 will continue to work without changes.
• Dual compatibility: Supports both google_analytics_v4 (GA4) and google_analytics (UA) configuration variables in _config.yml.
• Simple configuration: Users can set one variables in _config.yml as needed. Even if they use both, in theory is will make sense: they may want to use it to track the page by two Analytics IDs.
• Extendible: In future, new versions will arrive. This pattern can accommodate other versions if we need to support it long-term.
• Brand agnostic: Other companies, non-google, solutions can use the same pattern.

Usage

In your _config.yml:

google_analytics_v4: G-XXXXXXXXXX   # For GA4
google_analytics: UA-XXXXXXXX-X     # For Universal Analytics

Note:

• To credit @bovlb's solution, I forked the PR by @bovlb in PR #134, so it will in the commit history and a joint merge commit.