Add ES6 modules to the curriculum somewhere

[illicitonion]

We've discussed wanting to introduce modules many times.

In particular, this is relevant to testing and being able to split tests across different files from implementation.

MigraCode Barcelona has some content at https://github.com/Migracode-Barcelona/js3-workshop#es-6-modules

@40thieves also has some draft content here.

There's also discussion of this in https://codeyourfuture.slack.com/archives/C012UUW69S8/p1699920931507129

[illicitonion]

We already introduce modules in JS2 - maybe there?

[40thieves]

@40thieves also has some draft content here.

Hope I didn't over sell this! I will have a look for an old branch somewhere but it's ~possible~ probable that it's long gone by now unfortunately.

[sztupy]

Note: the proposed new starter kit and full stack assessment now requires Node 20+ and has been upgraded to uses ES6 modules only (also, no babel anymore, phew). We likely need to update the Node module to convert it to ES6 as well.