Add basic JavaScript types

[annevk]

We should add undefined, null, and boolean (true, false). We haven't made much type-value distinction thus far so I'm not quite sure how to formulate this. Anyone ideas?

[annevk]

Split from #87.

[domenic]

I don't think we should have undefined in specs unless you are specifically dealing with JavaScript. And I think JavaScript should be a level above Infra.

I might just define the three values for now (inside the variables section?) without worrying about a type system.

[annevk]

It seems like it should be in "primitive data types", but maybe its own section called "primitive values"? Where would we define Number/BigInt?

[domenic]

Hmm yeah, primitive data types is probably better. Probably no need for a separate section. We'd definitely put number/bigint there.