feat: use .editorconfig values as fallback config source

I already have things like indent size configured in [.editorconfig](https://editorconfig.org). I'd love to have yamlfmt use those settings rather than duplicate them in `.yamlfmt`.

Would you be open to a PR that includes [editorconfig-core-go](https://github.com/editorconfig/editorconfig-core-go) and uses `.editorconfig` values as defaults?