Preview improvements

[alcarney]

The PR contains a collection of improvements to previews in both the language server and the VSCode extension

• The server will once again, automatically trigger a build if needed (Closes #783)
• The server will automatically react to changes in esbonio.preview.* options
• If the client supports it, the server will send window/showDocument requests when previewing a file
• The VSCode extension will now show a "No Content Found" message containing troubleshooting steps in the preview window, when appropriate
• The VSCode extension now adds a button to the toolbar for markdown files that opens the preview pane