Add a --quiet-upstream flag

[nex3]

Most users only have the capacity to fix deprecation warnings in their own code, but they may end up using libraries that have their own deprecation warnings. We should add a --quiet-upstream flag (and similar flags for the Dart and JS APIs) to silence deprecation warnings emitted by upstream libraries (that is, libraries loaded through load paths and package: URLs).

I think this flag should probably not silence warnings due to @warn, since libraries frequently use those to indicate that downstream users are using the library in an invalid or deprecated way.

[vladimiry]

Since 1.33.0 I stared to see a lot of deprecated division operation related warnings. Is there a workaround to disable the warnings? The module used via sass-loader.

[huineng]

a zillion messages because not only the deprecation warning, but also the complete stack

[nex3]

Prioritizing this because of all the warnings generated by the /-as-division deprecation.

[nex3]

Renaming this to --quiet-deps because A) it's shorter and B) some users may not understand what "upstream" refers to.

[nex3]

To be clear, though: this option is only available via the Sass CLI and the Dart API. It would be much harder to add to the Node API, because Node's importer model makes it much harder to track what counts as the "main package" and what counts as a "dependency". We'll be able to support in the new JS API (sass/sass#3056) and in the embedded host, but those will take some time.

In the meantime, Node users will still see fewer deprecation warnings in the next release due to #1322, which skips warnings that have already been emitted for a given source location. That should help mitigate a substantial amount of the noise.