dsh-gitbash: Installing a Real Bash Tool for DSH on Windows

The `dsh-gitbash` plugin for DSH is designed to address the complex escaping issues that arise when invoking Bash commands via PowerShell on Windows. By using an “isolated scope” technique, it runs Git Bash independently within DSH without requiring preset switching or additional escaping. The plugin works without modifying existing presets and directly provides a `bash` tool after installation. It supports specifying the Bash path via a configuration option and remains compatible with upstream configurations. It is available only on Windows, executes commands with DSH’s own process privileges, operates without interference, and does not modify system environment variables.