Add basic Julia syntax material

[graeme-a-stewart]

Go quickly through variables, operators, functions, control flow.

This is to make the material more complete - it should be reasonably fast to get through and serve as reassurance/reference for learners.

I try to put in enough ~Easter Eggs, bits of content that show off Julia's features, that it's interesting for people and not too dry.

[graeme-a-stewart]

Forgot to add that I also did a specific "installing" notebook that covers installation, REPL, notebooks, scripts.

I decided to go for "classic" installation methods, with juliaup as an advanced option, rather than the default.

[aoanla]

Ah, I didn't see this before I did my bit. Let me look at your PR and my PR and merge manually!

[graeme-a-stewart]

Now subsumed by #8