Reusable split code editor for markdown

[chrisgregan]

Redesign the markdown editing system to make it easier to implement similar editors for other formats (e.g. AsciiDoc).

Redesign the markdown editor to provide preview, split and source edit modes in the same window as the text editor. Change markdown preview to use marked.js running in a WebView2 rather than performing the Markdown rendering on the C# side. Add a resuable SplitCodeEditor with JSON-RPC interface.

This redesign will also align all the document views to work in the same manner using JSON-RPC, so we can then move on to adding more types of editor view.