Fail dist build when for postcss warnings

[dgreif]

What are you trying to accomplish?

We would like to detect postcss warnings at build time, and prevent them from being merged/published

What approach did you choose and why?

Throw an error when a warning is detected during postcss build. Sample output from a few commits back:

@dgreif ➜ /workspaces/css (37deef3a ✗) $ script/dist.js
autoprefixer: /workspaces/css/src/layout/index.scss#sass:586:2: start value has mixed support, consider using flex-start instead
Error: Warnings while compiling src/layout/index.scss.  See output above.
    at file:///workspaces/css/script/dist.js:60:15
    at async Promise.all (index 17)
    at async dist (file:///workspaces/css/script/dist.js:72:5)
autoprefixer: /workspaces/css/src/core/index.scss#sass:1:0: start value has mixed support, consider using flex-start instead
autoprefixer: /workspaces/css/src/index.scss#sass:10455:2: start value has mixed support, consider using flex-start instead

This will cause script/dist.js to exit with code 1

What should reviewers focus on?

Is failing the build for warnings the right approach?

Can these changes ship as is?

• Yes, this PR does not depend on additional changes. 🚢

[changeset-bot]

🦋 Changeset detected

Latest commit: ca2d3ac

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@primer/css	Patch

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