This is the plugin MarkdownLivePreview. I have recently (16 November 2019) rewritten everything so that it is easier for people to contribute.
Hopefully, I'll get around writting a blog post about it soon.
To install it, it's on Package Control
To open the preview, just open a markdown file, and in the command
palette, you should find MarkdownLivePreview: Open Preview
If you want to use a keybinding, just add this to your keybinding file
{
"keys": ["alt+m"],
"command": "open_markdown_preview"
}
FIXME: upload a quick demo gif. (wanna send a pull request?)