Add new fullstack template (react+express) and support template options

[senko]

The new template:

• uses react with tailwind css and shadcn components on the frontend (using vite dev server)
• uses node/express backend with an optional sql (via prisma) or mongodb (via mongoose) database backend
• has optional authentication support (frontend and backend)

To support this, the PR also refactors the template engine to allow for separate template selection, architectural details selection, template configuration/summarization as well as ground work for future multiple templates support.