Add "env set-from-file" command to CLI

[C41M50N]

Add "env set-from-file" command to CLI so that users can set multiple deployment environment variables in one command.

# Before
npx convex env set CLERK_SECRET_KEY sk_test...
npx convex env set FIRECRAWL_API_KEY fc-...
npx convex env set OPENAI_API_KEY sk-proj-...
npx convex env set GEMINI_API_KEY sk-gem-...

# After
npx convex env set-from-file .env.prod

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