dsh-timezone-diff: Time Zone Difference Plugin

The DSH plugin `dsh-timezone-diff` provides a local time-difference calculation solution. Implemented in pure Node.js, it relies on no network requests, and its core tool `timezone_diff` supports precise parameter-based calculations. Install it with `dsh plugin add github:uckkk/dsh-timezone-diff`. It is suitable for scenarios that require handling cross-timezone time conversion within the DSH ecosystem.