[js/dotprompt] Partials disappear when developing with Next.js hot reloading

[mbleigh]

This has quickly become the most annoying bug in my Genkit workflow...when defining partials along with Dotprompt, the partials are disappearing from the Handlebars instance frequently (every hot reload perhaps?) when using the Next.js dev server.

I think the fix for this could be implemented along with #566 if partials were essentially re-bootstrapped on every prompt invocation when NODE_ENV=development or GENKIT_ENV=dev.