Plan for 4.x version of Google Analytics module

Problem/Motivation

We currently have 2 versions of the Google Analytics module, with 2.x covering the legacy analytics.js version, and 3.x using gtag. After conversation with Google, it seems important that this module has support for both, and that the type of .js is different than the version of GA that is being used.

Also, the existing module was mostly a port from D7, and thus contains most of its code within .module. This doesn't allow for a pluggable system where a user can choose between gtag and anayltics.js.

Lastly, as D9 comes to more wide use, its suggested we drop 8.x from the versioning. This requires a major bump, so this is probably a good time to do it.

Proposed resolution

Create a new 4.x version that supports both Anayltics and Gtag modules. Create a plugin manager that allows a user to choose between the two types.
A user should be able to upgrade from 2.x and 3.x to 4.x without data-breaking issues. However, sites that modify the existing 2.x or 3.x codebase or extend it will need to update their custom code.