feat: streamable stdout

[Kotablip]

Hello, this PR adds a new function in jbang.py that returns a reference to a subprocess.Popen instance, so that stdout can be inspected during the execution of a script.

The inspiration for this is that I needed to make some heavy database calls in Java, and I wanted to be able to track the process in my Jupyter notebook (which was later used for data wrangling).

Note:
I removed the source part of the activation commands in .justfile, because otherwise I was met with this error (on Ubuntu):

source .venv/bin/activate
sh: 1: source: not found
error: Recipe `test` failed on line 2 with exit code 127