Offer a way to open an untitled didact file via the extension_name:file_name Uri

[bfitzpat]

An issue has come up in the VS Code Development slack.

An extension developer has created an extension that edits markdown documents loaded from a backend server but has run into issues editing untitled documents. One of the things explored was using Didact to render them, but currently it can't process them.

From the conversation:

Hm for example in our extension (linked before Bugout) I load files from server and open them as markdown in editable mode. During user work with them I send updates to server with function onDidChangeTextDocument so they always synchronized. But I can't implement additional features, because this is dirty file, I can't add special buttons (for example as in microsoft/vscode-extension-samples/virtual-document-sample the button "Cowsay") and so on.. This untitled documents is so limited until they will be saved.