Add env file support

It would be nice to add an option for importing environment variables from an env file.

In integration tests it's a common pattern to use tokens and access keys from the environment and skip tests when they are missing. In local development environments this is commonly solved by using env files.

Although it's relatively easy to use godotenv for that, it adds an extra dependency, hence more complexity to the environment, so I think it would be a good fit for gotestsum.