Hello John,
Let me to think about. The objective was to maintain the backward compability, so, by default the "Is Content" and "Is background" checkboxes are unchecked, means the script that we add (or we already add in the past) are used by the app context, and not by the Web Extension context.
What do you want to share between the app context and the Web Extension content context and Web Extension background context? Remember that we are talking about different contexts, so, I am not sure if we can really use the same stuff in all contexts.
In fact, I am not sure if we really can do it... let me to explain. Supose you have a script which is used by the app context. That stuff is linked in the app's HTML, in order to be available in the app context, but, this stuff is not available in the Web Extension context, and cannot be available...
At least cannot be available "as is"... supose you place a global variable in the app's script, then you cannot use that global variable in the Web Extension context... because we are talking about the same script, but, the script is not shared between the different contexts, so possible changes in the variable in one context, are not available in other context.
If what you wanted is to use some JavaScript object definition, in order to be available in all the contexts, then, a possible solution right now can be to duplicate the script, that is, you can prepare two "script1.js" and "script2.js", with the same JavaScript object definition, so you can use the app's File Manager and add the two scripts, one for the app context, and the other for the Web Extension content and / or background context.
Remember the ability to share stuff between the app and the Web Extension contexts using messages: this is the way that we must use in order to communicate some possible variables values, etc. Adding the same script in the app and the Web Extension contexts cannot avoid the requirement to use messages in order to communicate the different contexts that we are talking about.
Please, post here if you have any question or want to comment something about, John.