GH-83417: Allow `venv` to add a `.gitignore` file to environments via a new `scm_ignore_file` parameter (GH-108125)

This feature is off by default via code but on by default via the CLI. The `.gitignore` file contains `*` which causes the entire directory to be ignored.

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>
Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>