Open different shell in same directory

# Description of the new feature/enhancement

Sometimes, it so happens that I am in (lets say) powershell, and i am working on something, and i change directory to some location, and i find myself in need of a different shell for some reason. Just as an example, maybe i have to run a bash script present in that directory using Git Bash / Msys.

Asked this on [reddit](https://old.reddit.com/r/programming/comments/nkrwnf/windows_terminal_preview_19_release/gzi0qyj/) and @zadjii-msft asked to open an issue here

# Proposed technical implementation details (optional)

No idea

The issue does not name any files, tests, or entry points. Start by reviewing how Windows Terminal launches and switches between shells, including PowerShell and Git Bash/MSYS, and determine how the current directory is represented. Done would require an agreed design and an implementation that opens another shell in the same directory.

Written by the indexing model from the issue text.