Add support for a split view preview when editing ytt templates

VScode natively supports split view previews for markdown, maybe we can implement something similar for ytt templates. We should be able to use the supported [WebView API](https://code.visualstudio.com/api/extension-guides/webview) to accomplish this though, we would need a way for the user to enter a custom command or somehow specify what files ytt should be run with.

The issue names no repository files or tests. Start by reviewing the VS Code WebView API and the existing split-preview behavior it references for Markdown; define how users specify the ytt command and input files, then verify that editing a ytt template can show its rendered preview side by side.

Written by the indexing model from the issue text.