Demos folder should allow regular component co-location

[NullVoxPopuli]

Related: #123

for example:

topic/
  demos/
     my-demo-a.md

in my-demo-a.md, if we have:

```hbs template
// ...
```
```js component
// ...
```

we should also be able to have:

topic/
  demos/
    my-demo-a/
      index.hbs
      index.js

or

topic/
  demos/
    my-demo-a.hbs
    my-demo-a.js

motivation being here is that we get more native tooling for editors that don't yet have (or have good) support for embedded languages.

Additionally, this would allow folks to have any module co-located with their demos (which could be useful for sample data)