Support sourcing environment variables from `.env` files

Some project support loading environment variables from `.env` files to allow permanent variables to be committed and shared among project members. Examples include [`create-react-app`](https://create-react-app.dev/docs/adding-custom-environment-variables#adding-development-environment-variables-in-env) and [Docker Compose](https://docs.docker.com/compose/env-file/). I think it would be great if `yarnhook` did as well. That way, a project team could share a variable such as `YARNHOOK_DRYRUN=true` without requiring each member to set it in a global `.bashrc` file.

Though this may seem to limit individual preference in environment variables, `yarnhook` could also recognize `.env.local`, which overrides `.env` and is usually listed in `.gitignore`

Start by tracing how yarnhook currently reads environment variables, especially YARNHOOK_DRYRUN, and identify its command-line entry point. Review the linked create-react-app and Docker Compose examples for expected .env behavior, including .env.local overriding .env. Done means shared variables load from .env while local overrides remain possible.

Written by the indexing model from the issue text.