gh-94214: Add venv context.lib_path and document the context (GH-94221)

[pfmoore]

Fixes #94214.

This adds the libpath of the new virtual environment to the context object returned by ensure_directories, and documents the context object so that callers can use it to introspect the newly-created environment.

I haven't added tests for this, but should we do so as we're now documenting the context?

• Issue: Expose libpath in venv.EnvBuilder.ensure_directories return value #94214

[pfmoore]

Awesome! Thanks for the quick review and merge 🙂