Logger support for WARN level

[jace-ys]

Just out of interest, why does the clue log package not provide support for logging at WARN level?

Seems like most packages like https://pkg.go.dev/log/slog have support for the main 4 log levels: DEBUG, INFO, WARN, ERROR.

Would be great to include support for WARN in clue log! I'm happy to open a PR if I get the greenlight.