Updating dotenv load and loadSync with assertion + type inference

[harrysolovay]

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

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
}