It's now possible to assign expressions to variables. The variables are
put into the current scope of the interpreter.
Variable lookup follows the scope chain, ending in the global object.
I always tell people to start building things by working on the thing
that seems the most interesting right now. The most interesting thing
here was an AST + simple interpreter, so that's where we start!
There is no lexer or parser yet, we build an AST directly and then
execute it in the interpreter, producing a return value.
This seems like the start of something interesting. :^)
