Add EXPORT functionality

As a user, I'd like to be able to use this as a normal environment loader as well if I so choose, such as using eval in front of it, so that the variables get the export keyword prepended.

Say I have many environment files that I switch between for local development, they may not contain secrets, but I don't necessarily want to manage them with generic Linux env files, and I'd like to be able to put the yml in VC for other teammates to replicate that non sensitive environment.

While using source is an option, if I'm going to use nv for secrets management as well, it may make sense to just use one tool.

I'm sure there are other applications to doing it that way as well or having that option, as I've used https://github.com/comcast/buildenv-tool with Makefiles this way for repeatable deployment and development as well.

The issue does not name a file, test, or CLI entry point. Start by tracing the command's environment-loading path and determine how an export-compatible output mode should interact with eval or source; done means the requested mode produces usable exported variables without changing the existing loader behavior.