Ensure default modules are loaded regardless of setting registration

[felixarntz]

Summary

Fixes #247

Relevant technical choices

• By passing the default as second parameter to get_option() in perflab_get_module_settings(), it is ensured the default value is always used, even when the setting is not registered yet.
• Since the plugin consistently relies on the perflab_get_module_settings() function to get the setting value, this ensures the default is also taken into consideration early (e.g. when the active modules are being loaded).
• The default is still also being registered, which is still necessary to support potential other plugins that may need it, e.g. when calling get_option() directly.
• Tests are included to verify the expected behavior.

Checklist

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

[mitogh]

LGTM 🎉

[felixarntz]

I've tested this PR on a fresh site and can confirm that all non-experimental modules are now loaded there right away (i.e. without updating settings under Settings > Performance), when before they weren't:

• WebP images are being generated.
• The Site Health check for WebP shows up.
• The Site Health check for a persistent object cache shows up.