Replace unreliable activation hook with default value for enabled modules

[felixarntz]

Summary

Fixes #218
Fixes #219

Relevant technical choices

• The usage of plugin activation hooks is universally problematic when it comes to environments like multisite or tools like WP-CLI, since the hook may not get triggered, or in multisite it would only be triggered on the main site when network-activating the plugin.
• Therefore this PR replaces it with using a default value for the option.
  • The straightforward implementation here would be to either hard-code the list of non-experimental modules in there or to dynamically read them from the filesystem, however both of that is not desirable: Hard-coding would result in manual maintenance which would be easily overlooked, and dynamically reading from the filesystem would be bad for performance since this would then need to run on every page load, even in the frontend of the site.
  • Because of the above, this PR uses a separate PHP file default-enabled-modules.php which, similarly to the existing module-i18n.php file, can be automatically generated and maintained.
• For the above, the PR implements a new CLI script similar to some of the existing ones, which allows to refresh the new auto-generated PHP file with a single command.
• The release instructions are being expanded accordingly to include running this new command before every plugin release.

Checklist

• PR has either [Focus] or Infrastructure label.
• PR has a [Type] label.
• PR has a milestone or the no milestone label.

[gagan0123]

Tested scenarios,

1. Multisite
   1. Activate on single site
   2. Network Activate
2. WP-CLI install and activate in single and multisite
3. WP-CLI activate in single site

All scenarios now working as expected with default values as they should be.