feat: support env vars in dev command

[HenryLee789]

Summary

• add --env-var/-v and --env-file/-f support to mcp dev
• share environment collection logic with mcp install
• document env var usage in the README and add CLI helper tests

Fixes #339

Testing

• uv run pytest tests/cli/test_utils.py -q
• uv run ruff check src/mcp/cli/cli.py tests/cli/test_utils.py

[maxisbey]

Closing as a duplicate of #2506, which was opened first with the same feature. Thanks for the contribution.

_{AI Disclaimer}