Support for parsing structured headers for AOs and similar operations

[bakkot]

As in tc39/ecma262#545.

Headers look like this:

<emu-clause id="sec-exampleao" type="abstract operation">
  <h1>
    ExampleAO (
      param : an integer,
      optional param2 : a String,
    )
  </h1>
  <dl class='header'>
    <dt>description</dt>
    <dd>It is an example.</dd>
  </dl>
  [...]

and produce HTML that looks like this (modulo formatting and other details):

<emu-clause id="sec-exampleao" aoid="ExampleAO">
  <h1>ExampleAO ( param [ , param2 ] )</h1>
  <p>The abstract operation ExampleAO takes argument param (an integer) and optional argument param2 (a String). It is an example.</p>
  [...]

Of note:

• the name of the AO now appears in the original source once, instead of three times
• the name of each parameter now appears in the original source once, instead of twice
• there is a new dd element which we can stuff arbitrary other machine-readable attributes into

This is not a breaking change, since the previous format is still supported.

[michaelficarra]

LGTM otherwise.