[mdx] Bundle size

[pomber]

Two improvements to bundle size:

• only include the theme once
• import only the components that are being used

For example, in:

# Hello

```js
function lorem(ipsum) {}
```

```js
function lorem(ipsum) {}
```

The theme should appear only once in the output. Also, unused components (like CH.Spotlight) shouldn't be included in the final bundle.

[pomber]

Fixed with #160