Updating dotenv `load` and `loadSync` with assertion + type inference

Thoughts on updating the `load` and `loadSync` functions such that an `assertExists` option could be supplied (which would then inform the returned env record)?

```ts
import { load } from "@std/dotenv"

const env = await load({
assertExists: ["STRIPE_API_KEY", "OPENAI_API_KEY", "DATABASE_URL"]
})

env satisfies {
STRIPE_API_KEY: string
OPENAI_API_KEY: string
DATABASE_URL: string
}
```