Clarify compile-time errors

[elibarzilay]

An example like

let foo = "hello!";
foo();

would gets newbie readers very confused: if all you do is "run the code", then doing so with plain JS or with TS gets you an error anyway, so what's the big fuss? Clarify this in the text, and also add a function so that plain JS wouldn't complain, to clarify it further.