Vertically align markdown in markdown preview

[darkclouder]

So far, the markdown is on the top line. Especially for a presentation I think though vertically putting the markdown in the centre of it does not fill the screen would be nice.

[nicoespeon]

That would be nice indeed. It will be tricky to do with the current implementation though since we are delegating the work to VS Code and it doesn't support this feature: https://github.com/microsoft/vscode/blob/f44dc0853786a1a1c9f5dce5cd94941c2a795655/extensions/markdown-language-features/src/commands/showPreview.ts#L61