Improved convex env workflow

npx convex env set now accepts values interactively, via --from-file, or multiple variables at once via stdin (e.g. npx convex env set < .env.defaults). npx convex env list emits a robust format for multi-line variables, enabling round-tripping between deployments with npx convex env list > .env.convex and npx convex env set < .env.convex.