feat: support selective log flushing

[xalonious]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	None
License	AGPL-3.0
Doc PR	N/A — README updated

Summary

PM2 already supports selecting stdout and stderr independently with pm2 logs --out and pm2 logs --err. This change adds equivalent stream selection to pm2 flush:

• pm2 flush <app> --out clears only stdout logs.
• pm2 flush <app> --err clears only stderr logs.
• When both flags are supplied, --err takes precedence, matching pm2 logs.

Existing flush commands continue clearing both streams. The programmatic API remains backward-compatible.

Flushing uses existing process log metadata, supporting custom paths, application names, process IDs, and cluster workers. Selecting either stream clears a shared stdout/stderr file.

[CLAassistant]

All committers have signed the CLA.