Showcase mode: replace client-side markdown rendering with `showdown` with server-side rendering

[gadenbuie]

#4201 brought to my attention that we're using client-side rendering via the showdown JS library to render an app's README.md file in show case mode.

shiny/srcts/extras/shiny-showcase.ts

Lines 280 to 294 in f55c26a

	function renderMarkdown() {
	const mdContent = document.getElementById("showcase-markdown-content");
	if (mdContent !== null) {
	// IE8 puts the content of <script> tags into innerHTML but
	// not innerText
	const content = mdContent.innerText || mdContent.innerHTML;
	const showdownConverter = (window as any).Showdown
	.converter as showdown.ConverterStatic;
	document.getElementById("readme-md").innerHTML =
	new showdownConverter().makeHtml(content);
	}
	}

We now have a strong dependency on commonmark and can handle this rendering server-side, avoiding the additional JavaScript dependency.