[HTTP] Add pretty and preview to response bodies

[jcoc611]

Requirements

For HTTP responses:

• A pretty subtype is added to the body component, which uses a formatting and syntax-highlighting similar to VSCode.
• A safe preview subtype is added to the body component, which renders the response content without downloading any extra resources and without running JavaScript. For now, only text/html rendering is needed, but later could also add images, CSS and JavaScript.
• It makes sense to make pretty the default (first) subtype

[jcoc611]

preview added using <iframe sandbox ...> in v0.2.3. Probably good enough for now. pretty will be added later, probably once the Notebook API settles down on VS Code, and I can embed the native editor in the client webview.