Reading .env or config.edn files by default

As a new user I've been struggling with this for a while. From what I can tell, `system.edn` uses aero to read environment variables, but it has no ability to read from actual .env files.

Since Integrant is so central to the kit workflow, it'd be great if there was already a bootstrap in place for reading from either .env or config.edn files. It's standard to have local environment files that aren't in version history. I, however, have had trouble finding a way to "export" these locally defined variables before integrant's config is read.

There doesn't appear to be anything for this at the moment.

Start by inspecting system.edn and the Integrant configuration-loading path, including how aero currently reads environment variables. Determine where .env or config.edn values would need to be available before Integrant reads its config; done means a documented default bootstrap supports those local files without requiring manual exports.

Written by the indexing model from the issue text.