Add highlight.js support for text/event-stream

[handrews]

This is relevant for a soon-to-be posted OAS 3.2 PR. See Discussion #4171.
MDN has much more readable text/event-stream documentation than WHATWG.

• schema changes are included in this pull request
• schema changes are needed for this pull request but not done yet
• no schema changes are needed for this pull request

[ralfhandl]

@handrews Would you mind adding this to the test fixture and the expected output, similar to

OpenAPI-Specification/tests/md2html/fixtures/basic-new.md

Lines 61 to 64 in beb56e0

	```uritemplate
	https://foo.com/bar{?baz*,qux}
	```

and

OpenAPI-Specification/tests/md2html/fixtures/basic-new.html

Lines 46 to 47 in beb56e0

	<pre class="nohighlight" tabindex="0"><code>https://foo.com/bar{<span class="hljs-attr">?baz*</span>,<span class="hljs-attr">qux</span>}
	</code></pre>

That way you can verify locally in the browser whether the expected output has the desired visual appearance.

Just

• Add new code block to basic-new.md
• npm test shows expected (and missing) HTML snippet in red
• Copy expected HTML snippet from test output to basic-new.html
• npm test shows green (if you copied correctly)
• View basic-new.html in browser to verify visual appearance
• Rinse and repeat