Add support for markdown rendering

[mattesmohr]

This PR adds support for markdown rendering, limited on inline markdown such as text emphasis, code or links - similar to SwiftUI.

Paragraph {
   MarkdownString("This is a *italic* and **strong** emphasis.")
}

<p>This is a <em>italic</em> and <strong>strong</strong> emphasis.</p>

For the time being, the markdown rendering will be hidden behind a feature flag. So if you would like to use it, you need to enable it first.