Add `--verbose` flag and reduce default logging

[khoadng]

Motivation
Slang’s default CLI output is a bit too noisy. It clutters the console logs especially when running multiple scripts in parallel.

Developer Experience

• Default: minimal logging (only essential info + errors).
• --verbose flag: opt-in for detailed logs (current behavior).
• (Optional) --silent flag to suppress all non-error output.

If you agree with this direction, I’m happy to implement this and submit a PR.

[Tienisto]

Sure!