feat(webpack-cli): add mode argument validation

[snitin315]

What kind of change does this PR introduce?

Fixes #1282

Refers #1261

Did you add tests for your changes?
Yes.

If relevant, did you update the documentation?
Yes

Summary
Added validation for mode argument , now can only accept development , production or none
will promt a warning if set to any other value.

Does this PR introduce a breaking change?

NO.

[webpack-bot]

Thank you for your pull request! The most important CI builds succeeded, we’ll review the pull request soon.

[ematipico]

Please change the test. I we should not say anything about falling back. Webpack already does that and it prints a warning too.

[webpack-bot]

@snitin315 Thanks for your update.

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

@ematipico Please review the new changes.

[snitin315]

I think we should add the test case for invalid value , as now we are returning production in this case.

@ematipico what do you say?

[ematipico]

Yes of course.
We still have a failing message about the snapshot. It's about your previous pr. Could you check that? I believe the issue is windows ending line

[ematipico]

@snitin315 I fixed the failing tests. Would you please rebase your PR?

[snitin315]

@ematipico Done 💯

[ematipico]

We're almost there!

[ematipico]

Thank you!