Support sourcing environment variables from `.env` files

[0x326]

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 and Docker Compose. 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

[frontsideair]

I think this is an easy change to make, but I wonder if it warrants adding a dependency. Current environment flags act like one-off settings and I can't see a team using a .env file to set them project-wide.

On the other hand, in case of YARNHOOK_DRYRUN flag, if this is actually used by a project to only suggest reinstalling dependencies, we can use a configuration file (maybe cosmiconfig) and make it configurable this way.