[feat] Render to dotenv-style formats

[linux-china]

.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

[LloydNA]

I'll work on this issue

[bioball]

FYI--it's possible to define an in-language renderer; a renderer doesn't need to be part of the standard library.

For an example of an in-language renderer, see pkl.toml

[jtagrgh]

@LloydNA Is this currently a WIP?