Add --quiet-deprecation-warnings flag

[peteduel]

What is the problem you're trying to solve?

A page of deprecation warnings on v15.x

What solution would you like to see?

The quiet flag (or some other means) to opt-out of the warnings

[Mouvedia]

related discussion: #6561 (comment)

[ybiquitous]

@peteduel @Mouvedia Thanks for the report and for sharing the related discussion.

The current quiet option handles only linting problems with a severity (error or warning). On the other hand, deprecation warnings are not linting problems, so expanding the current option semantics may break something.

If we want to avoid such a breaking possibility, we may need to add a new option to suppress deprecation warnings.

What do you think? @jeddy3

[jeddy3]

If we want to avoid such a breaking possibility, we may need to add a new option to suppress deprecation warnings.

We could accept a PR for a --quiet-deprecation-warnings flag. It's explicit and clear.

[Mouvedia]

Should we add a disclaimer for the SKIP_WARNINGS environment variable in the documentation?

[ybiquitous]

@jeddy3 Thanks for the answer. Yes, adding a new flag may be safe. 👍🏼

@Mouvedia Where does that environment variable come from? I believe we have not used any environment variables in Stylelint... 🤔

[Mouvedia]

@ybiquitous you are right.
I misread

stylelint/docs/developer-guide/formatters.md

Line 71 in c1223a4

You can use environmental variables in your formatter. For example, pass `SKIP_WARNINGS`:

[jeddy3]

I've labelled the issue as ready to implement. Please consider contributing if you have time.