Just strings and numbers for now

I think there’s a better way to do type checking so I’m going to hold off on it for now and get basic functionality working for now and come back later.

This class exposes two functions. `evalNode` take an AST node and evaluates it down to simplest terms. `eval` takes an array of AST nodes and calls evalNode on each of them. I don’t think we’re quite ready to be producing HTML yet so let’s not get ahead of ourselves.

Currently I'm relying on JavaScript's semantics
around truthiness. I think I'm going to retain
that ease of use (i.e. `(if list #t #f)` should
return true if the list has more than 0 elements,
etc) but I want to specify that behavior myself.

Just numbers and strings for now

I noticed that if you passed in just a number, JS
would helpfully parse that and then
`source.length` was undefined, which broke the
lexer.

I keep test .oslo files around and I hate not
having a clean repo.

I added upper and lowercase because that seemed
preferable to always casting every html function
name to lowercase.

It's a little hacky and not very clean but it's
working for now.

The way that we were doing it before was going to
make it very difficult to implement any sort of
error handling or extension to the language.