[feat] Render to dotenv-style formats

`.env` accepted by lots of frameworks, and easy to be converted to environment variables.

For example, pkl as following:

```
name="joe"
server {
host = "localhost"
port =8080
}
```

and the pkl could be rendered to dotenv-style:

```
NAME="joe"
SERVER_HOST="localhost"
SERVER_PORT=8080
```

Start by locating the Java implementation and entry points for existing output renderers, then inspect how nested properties and scalar values are serialized. Use the examples in the issue as the expected dotenv-style output, including uppercased names and underscore-separated nesting; done means the new format renders equivalent values correctly.

Written by the indexing model from the issue text.