Finalize syntax (e.g., ```js {show}) to denote live code blocks

[mbostock]

We currently use standard fenced code blocks

```js
1 + 2
```

This can be surprising because it behaves differently from standard Markdown: the code runs instead of being displayed! And also it’s inconsistent with non-JavaScript code blocks, which do display as normal.

So instead, we could follow Quarto’s example by using curly braces:

```{js}
1 + 2
```

I also thought about using script tags, or some such…

<script type="observable">
1 + 2
</script>

But that seems weird. 🤔

[mbostock]

Another option would be markdown-it-container, which might look like this:

:::js
1 + 2
:::

[mootari]

Curly braces seem like the safest choice to me, not only because they follow Quarto's style but also because code blocks would still display as code (albeit without the correct highlighting) when viewing the Markdown files directly.

[cinxmo]

Does this ticket include which options we will support for Milestone 1: Internal Release?

[mbostock]

They’re not currently called out, but I believe all we need to support is:

• echo - whether to echo the code into the page with nice syntax highlighting; defaults to false if evaluate is true, and true if evaluate is false
• evaluate - whether to run the code; defaults to true
• display - whether to implicitly display the value for expression fenced code blocks, and/or whether to suppress the output from calls to display

The names above aren’t finalized, but I think that’s all the functionality we need. (Plus the ability to control at the page level via front matter but that is tracked separately.)