[feat] module to import appropriate .env

It's great using azd to provision resources and being able to load the _.env_ files using godotenv or whatever, but it would be even easier in projects using azd this way if we provided a library to make it easier e.g.,

```go
func LoadEnv(env string) error {
if env == "" {
// get .azure/config@defaultEnvironment or err
}
// use godotenv to load environment's .env file,
// optionally supporting fallback to/inclusion of any repo/ancestor .env file
}
```

It's not hard code to write, but any module support for azd (in Go or other languages) might help adoption.