Implement basic support for Markdown alerts in HTML-to-spanned conversion

This PR adds basic rendering support for [Markdown alerts](https://github.com/orgs/community/discussions/16925), a special type of blockquote that has been around for one year now. The appearance is kept intentionally simple (no colors, no title icons) to avoid dealing with different color tones for dark/light themes and to make them visually consistent with how they appear in the WebView viewer, where they are displayed as simple blockquotes because the Markdown JS library we use doesn't yet support them. | Before | After | |--------|--------| |  |  |