dsh-workspace-env: Inject DSH pwsh subprocess environment variables from workspace .env

`dsh-workspace-env` is a DSH plugin designed to address environment variable management issues in the Windows PowerShell (`pwsh`) environment. It achieves this by enhancing the `pwsh` executor to automatically read the `.env` file in the current workspace directory during each tool invocation and inject its variables into child processes, thereby realizing project isolation and avoiding issues like cross-contamination or the need to restart processes for changes to take effect. Its core features include automatically switching environments per workspace; real-time reading of `.env` on every call with immediate effect upon modification; support for `${VAR}` variable references and append operations (e.g., PATH prepending); automatic handling of quotes and comments; and strict ignoring of `DSH_*` prefix variables to prevent conflicts.