Add env:add command to define environment variable and its validation rules

• Merge pull request #4533 from adonisjs/feat/set-env (c8251b5)
• fix: fix breaking test (d80d7bf)
• refactor: defineEnvVariables with omitFromExample (9cd7183)
• feat: insert empty example in the .env.example (5a1a4e9)
• feat: add env:add command (4508128)

Full Changelog: v6.9.0...v6.9.0