Test shell integration proposed API

[anthonykim1]

Python extension should test out new shell integration's proposed API in Terminal and/or REPL.
Mainly for the purposes of terminal environment variable activation(sending activation commands, being able to print and see user's environment variables for complete activation and better PS1 handling), and better context of when/if Python terminal REPL launched.

[anthonykim1]

Moving this from August milestone since this itself was to test an API.

[anthonykim1]

Closing this as we are now actively using shell integration's now finalized API + executeCommand&commandline.execution, etc