feat: provide support for .env files injects via API

[easymikey]

Fixes #975

/// './path/prod.env'
FOO=BAR

---
/// index.js
$.env = './path/prod.env'
await $`echo $FOO`.stdout // BAR

• Tests pass
• Appropriate changes to README are included in PR

[easymikey]

[antongolub]

What's about this way instead?

const loadDotenv = (...files: string[]) => files.reverse().reduce<env>((m, f) => Object.assign(m, parseDotenv(fs.readFileSync(f))), {})

$.env = loadDotenv('.env', '.env.default')

[easymikey]

What's about this way instead?

const loadDotenv = (...files: string[]) => files.reverse().reduce<env>((m, f) => Object.assign(m, parseDotenv(fs.readFileSync(f))), {})

$.env = loadDotenv('.env', '.env.default')

Maybe replace it with spread?

const loadDotenv = (...files: string[]) =>
  files
    .reverse()
    .reduce<env>((m, f) => ({ ...m, ...parseDotenv(fs.readFileSync(f)) }), {});

And why the reverse?

$.env = loadDotenv('.env.default', '.env')

Are you suggesting we consider an array of strings as well?

type Env = NodeJs.Process | string | string[]

[antongolub]

I suggest keeping the $.env contract as is, and just export loadDotenv helper

[antongolub]

lgtm. Thanks.