Add Dotenv as its own language

[lildude]

This reverts commit 712a1ee.

Description

Add Dotenv as its own language and move the current .env and .env.example files from Shell to the new language.

I've only added samples for variants of the list in @Alhadis's grammar here where they're popular enough for inclusion now.

Fixes #5431

Checklist:

• I am adding a new language.
  • The extension of the new language is used in hundreds of repositories on GitHub.com.
    • Search results for each extension:
      • https://github.com/search?q=path%3A*.env.example&type=code
      • https://github.com/search?q=path%3A*.env&type=code
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      Unless listed below, these are samples already approved and included with Linguist
      • .env.ci
      • .env.dev
      • .env.development.local
      • .env.development
      • .env.local
      • .env.prod
      • .env.production
      • .env.staging
      • .env.test
      • .env.testing
    • Sample license(s):
      • .env.ci - MIT
      • .env.dev - MIT
      • .env.development.local - AGPL3
      • .env.development - MIT
      • .env.local - MIT
      • .env.prod - APL2
      • .env.production - MIT
      • .env.staging - MIT
      • .env.test - MIT
      • .env.testing - GPL2
  • I have included a syntax highlighting grammar: https://github.com/Alhadis/language-etc - we already use this for other languages.
  • I have updated the heuristics to distinguish my language from others using the same extension.

I've gone for the #e5d559 for the colour from the image at https://github.com/motdotla/dotenv

[spenserblack]

Just a heads up that .env.ci is a somewhat popular filename. It seems to be a bit more popular than .env.development.local.

[szepeviktor]

It is a break-through.

🎉

Thank you!