This makes it easier to add classes in a render method. Also updates the TemplateBuilder type to not require a Harness (ex: card does not need one)
PiperOrigin-RevId: 588110635
In particular, this allows external build tools like Rollup and @web/dev-server to understand imports of Lit, which is configured to require the ".js" extension via its export conditions (so that Lit import maps can remain minimal).
PiperOrigin-RevId: 469772992
