Upgrade to VS Code's webview API

[mjbvz]

Hi, I'm on the VS Code team. I noticed that your extension uses the vscode.previewHtml command which is deprecated and which we are actively working to remove: microsoft/vscode#62630

We've developed a webview API that provides a much better developer experience and offers a number of important security and compatibility benefits over previewHtml. We cannot fix previewHtml without breaking backwards compatibility, and have instead opted to remove it.

To ensure that your extension continues to work properly in VS Code, please try upgrading to use the new Webview API. You can find documentation on the API usage here. Let me know if you have any questions or concerns about this migration

[mjbvz]

Just a heads up: in VS Code 1.32 (the February update), using the previewHtml command will show an alert to the user, although the command should still work. We plan to remove the command entirely with VS Code 1.33 (March update)