python: Enhance startup script experience

Currently, users can create scripts that run at startup of a Python session by placing them in ~/.ipython/profile_default/startup/. There are some pain points with this process.

(See https://www.rdocumentation.org/packages/base/versions/3.6.2/topics/Startup for inspiration.)

• The scripts' output should be printed to the Console
• Non-hidden variables that are defined should be shown in the Variables pane without having to add an ipython config setting
• We could search default locations for the scripts, including at the Positron user and project levels. (Using the ~/.ipython directory means it's not just Positron that reads the scripts, and today there's no project-level customization)
• If that's insufficient, we could add a dedicated setting where users can select their custom startup scripts

Originally posted by @dani-f in https://github.com/posit-dev/positron/discussions/6376#discussioncomment-12313912

Start by tracing how Python session startup scripts in ~/.ipython/profile_default/startup/ are discovered and executed. Review the Console and Variables pane behavior, then determine how Positron user- and project-level locations would fit. Done means the intended startup locations or setting are defined, script output reaches the Console, and non-hidden variables appear in the Variables pane.