Add support for structured headers for SDOs

[bakkot]

Followup to #327 to extend support to SDOs. As in that PR, the old syntax is still parsed and treated just as today, so this is not a breaking change.

Unlike in #327, the HTML generated here is not identical to what's currently in 262. Specifically, 262 includes either no text at all, the text With parameters _foo_ and _bar_., or on a couple of occasions a paragraph of text. When using structured headers, the output will instead look like

The syntax-directed operation Name takes parameter foo. [optional description] It is defined piecewise over the following productions:

---

There's a slight oddity here: when using this style to specify parameters, you'd write e.g.

  <emu-clause id="sec-example" type="sdo">
    <h1>
      Example (
        _param_: an ECMAScript language value,
      )
    </h1>
    <dl class='header'>
    </dl>

However, the parameter list never appears in ( list ) form in the output, only in the "takes argument param" prose. I think that's fine, but it might be a bit confusing.

[jmdyck]

Why not include the parameter 'type' in the generated prose?

[bakkot]

Why not include the parameter 'type' in the generated prose?

It is included. See e.g. EvaluateAsyncFunctionBody.

[jmdyck]

Then I don't understand the "slight oddity" you were describing. Never mind, got it now.