refactor: add use imports for relocated Env and Exit effects

[magnus-madsen]

Summary

• Add use Sys.Env and use Sys.Exit imports to code blocks in src/main.md and src/default-handlers.md so short names resolve after the effects were relocated to Sys.Env and Sys.Exit
• No changes needed for Random (only Java interop refs), Process (already has imports), for-llms.md / hello-world.md (prose only)

Test plan

• Verify code snippets in src/main.md compile with the Flix compiler
• Verify code snippets in src/default-handlers.md compile with the Flix compiler

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