amp-script: Support d3.js

Our goal is to run JavaScript data visualization libraries using `amp-script` -- avoiding need to use `amp-iframe`.

This example implements `canvas.js` with `amp-script`
https://afdsi.org/amp-script-03/
Source is Weston Ruter and his experiment here:
https://github.com/westonruter/javascriptforwp-conference-amp-examples
using this technique:
```

Start drawing!

```
We would like to implement our `d3.js` applications using a similar technique.
A simple stand-alone D3 version is shown here:
https://afdsi.org/amp-script/reference.html

Our experiment follows `canvas.js` example, but uses a `d3.js` library as follows:
```

```
The experiment is here:
https://afdsi.org/amp-script-01/
We see the following error message:
```
Uncaught ReferenceError: d3 is not defined
amp-script[src="dendrogram.js"].js:10
at Object. (amp-script[src="dendrogram.js"].js:10)
at amp-script[src="dendrogram.js"].js:70
```
Are we doing something wrong?
Is there an alternative way to invoke a `d3.js` library?

We see these discussions:
https://github.com/ampproject/amphtml/issues/23579
https://github.com/ampproject/amphtml/pull/23658
https://github.com/ampproject/amphtml/issues/18830
https://github.com/ampproject/amphtml/issues/13471

Start by comparing the working canvas.js example with the d3.js and dendrogram.js amp-script examples, focusing on why d3 is undefined. Read the linked discussions and related issue and pull request for existing constraints. Done means the requested D3 usage path is resolved or its supported alternative is clearly established without the reported error.

Written by the indexing model from the issue text.