Add the ability to specify global & project-level default .env filenames

[jongpie]

Currently, the plugin supports specifying an .env file using the flag --env - this is great when running commands via a terminal, but when using VS Code (or other IDEs), there's not a way to use anything other than the assumed default filename .env.

• Users should be able to specify a global default filename, for situations where something other than .env is generally preferred
• Users should also be able to specify a repo/project-level default filename, so they can easily switch between different environments/environment variables (e.g., dev.env vs prod.env)

I think that the global default filename (configured on someone's machine) would live outside of source control - there might be some value in having project-level defaults be committed to source control, but probably it should also just live outside of source control 🤔