Preview in VSCode

[michaeltlombardi]

As a platen site maintainer, I want to be able to preview my changes locally in my editor instead of needing to refer between my editor and a separate browser window.

This functionality may be readily provided by the Front Matter extension through its site preview feature. Being able to do so reliably for theme users will require the implementation of
estruyf/vscode-front-matter#407, which would allow us to publish one (or more) base configurations that users could inherit and extend for their own sites.

Without that feature, we would need to implement a way to:

1. Ship the latest configuration to users when they use the template repository
2. Write a helper to enable users to add the latest configuration to their site if they had already created their own project (we will probably want to do this anyway for convenience)
3. Write helpers for handling configuration updating/downgrading/etc - possibly scripts, possibly GitHub Actions, probably both.