feat(cli): line mode option support

[ysfscream]

PR Checklist

What is the current behavior?

To read messages from stdin line by line, users need to use two separate options: -s and -M. This combination is not intuitive and requires users to remember to use both options together.

Example of current usage:

mqttx pub -t "topic" -s -M

What is the new behavior?

Added a new convenient option -lm, --line-mode that combines the functionality of -s and -M. This makes the command more intuitive and easier to use.

New usage:

❯ mqttx pub -t test -lm
✔ Connected, input and press Enter to publish
- Press Ctrl+C to disconnect and exit
test

The old way (-s -M) still works for backward compatibility, but the new option provides a more straightforward approach.

Does this PR introduce a breaking change?

• Yes
• No

Specific Instructions

1. The new -lm option is equivalent to using -s -M together
2. Existing code using -s -M will continue to work as before