ES6 Modules: Clarify examples won't work without next section

[MaoShizhong]

Because

People often like to code along to lessons. The ES6 modules lesson first shows import/export syntax, then teaches how to link the entry point as a module script. Since it doesn't make it clear both things are needed, some people may get confused when the import/export code errors. A quick clarification is all that's needed here.

This PR

• Clarify some code examples won't work straight away
• Suggest going back to rewrite IIFE examples to use ESM if people wish to "code along".

Issue

N/A

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[JoshDevHub]

🚀

[Salomanuel]

Just as feedback, even after this PR got merged I STILL had lots of issues understanding that the examples would never work as they were presented (what's the point of examples if they don't work? As a student you even lose trust in the process if the teacher "is wrong").

I had to look into GitHub to see if something was going on (and indeed there was! (this PR)).

I'd just state more clearly at the beginning of the lesson, something like "BEWARE FELLOW STUDENT, NONE OF THESE EXAMPLES WILL WORK AS OF NOW, so don't even bother coding along while reading" or just rewrite the whole thing so it starts making sense. :)