[css-animations-2] Add <dfn> reference for <timeline-name> definition

[pyoor]

The <timeline-name> syntax definition should include a reference to where it is used (i.e. <single-animation-timeline>) in the actual definition.

csswg-drafts/css-animations-2/Overview.bs

Lines 553 to 555 in 9c1c802

	<pre>
	&lt;timeline-name> = <<custom-ident>> | <<string>>
	</pre>

I'm not familiar with using bikeshed but the resulting HTML would look like:

<pre>
  <dfn class="dfn-paneled css" data-dfn-type="type" data-export="" id="typedef-timeline-name">&lt;timeline-name&gt;</dfn>
   = <a class="production" data-link-type="type" href="https://drafts.csswg.org/css-values-4/#identifier-value" id="ref-for-identifier-value">&lt;custom-ident&gt;</a> | <a class="production" data-link-type="type" href="https://drafts.csswg.org/css-values-3/#string-value" id="ref-for-string-value">&lt;string&gt;</a>
</pre>

This formatting would be consistent with other specs including the parent <single-animation-timeline> definition, and would also allow for easier parsing.