feat: 🎸 add support for env flag

[anshumanv]

What kind of change does this PR introduce?

feat

Did you add tests for your changes?
Yes

If relevant, did you update the documentation?
Yet to

Summary

• Add support for --env flag back.

Does this PR introduce a breaking change?
No

Other information

[alexander-akait]

Do we have docs about supported configurations? Like function/Promise/etc

[anshumanv]

@evilebottnawi yes - https://webpack.js.org/configuration/configuration-types/#exporting-a-function

[snitin315]

need to add this flag in docs as well https://github.com/webpack/webpack-cli/tree/next/packages/webpack-cli#webpack-cli

[alexander-akait]

Let's rename is to mode to avoid misleading between configuration and flags for CLI (also we already have mode flag)

[alexander-akait]

I was wrong, give me time to think

[alexander-akait]

I would like to avoid this because this is very misleading, Maybe we should just pass argv, without env flag (normalize them before)

[alexander-akait]

What is really diffidence between env and argv? I see no difference, just pass flags in difference ways

[anshumanv]

argv has other things too, env is only what passed to the configuration function.

[alexander-akait]

@anshumanv Alright, let's get this back for compatibility, but we will need to throw depications in the future and pass only args + normalized args

[webpack-bot]

@anshumanv Thanks for your update.

I labeled the Pull Request so reviewers will review it again.

@snitin315 Please review the new changes.