feat: quiet switch for harper-cli lint

[hippietrail]

Issues

I've wanted this for months. If it turns out I requested it I'll come back and fill this in.

Description

When working on linters, harper-cli lint on a file full of lintable files can be very handy. But every file with no lints, and every file with lines (technically) too long, would output messages, often filling up the terminal, leaving the dev to hunt for the actual lints among the output.

Those two messages will now be suppressed with the --quiet option.

How Has This Been Tested?

Manually and cargo test

AI Disclosure

• I am a human and didn't use any AI.
• I used LLM features of my editor, but not an agent.
• I used an AI agent interactively.
• I am an agent or I got an agent to do the work autonomously.

I did ask Google Search's AI for some tips.

Checklist

• I have performed a self-review of my own code
• I have added tests to cover my changes
• I have considered splitting this into smaller pull requests.