Improve setting up deactivate command in case of virtual environments

[karrtikr]

Part of #20950

Instead of asking users to edit their .bashrc, put the command in PATH so it's automatically picked up. This is currently blocked on microsoft/vscode#197187.

[karrtikr]

It's not ready to be verified until microsoft/vscode#197187 is fixed. Also blocked on microsoft/vscode#197768.

[karrtikr]

Verification steps:

• Use Powershell on Windows
• Create a Python virtual environment using Create environment command
• Open terminal, make sure .venv created is activated:

python -c"import sys;print(sys.executable)"

• Run "deactivate" command
• Print sys.executable again to make sure venv is deactivated