Updated `.prettierignore` to ingnore the typical Python environment files from `.venv`
If you are running pre-commit locally you probably have a Python environment setup.
This PR speeds up the prettier hook and avoids multiple passes through the targetted files.
Prettier uses an ignore file to exclude files from formatting when
running with pre-commit and also when running standalone from the command line.
This is detailed on the CLI docs page:
https://prettier.io/docs/cli