Extension: Support 'webview' functionality

[bryphe]

Several extensions are using VSCode's Webview API.

For example:

• foambubble/foam uses a webview to show a graph visualization
• golang.go uses a webview to create a welcome page, and show custom configuration

As extensions start to lean more heavily, we'll need to look at supporting this too. This is mainly just tracking for now, but something that is on our radar and we've discussed internally a bit.

In terms of supporting this, we'd need to decide between two options:

1. Use native webviews per-platform. Go's webview https://github.com/webview/webview has a nice model for this, but wouldn't be something we could drop-in because it doesn't support headless rendering or rendering to a buffer.
2. Use CEF which is more heavyweight than the native, but would use a common rendering platform.

• Could also be useful for Lack of HTML parsing for plugin descriptions #2499

Related #1058