Remove unitless calc addition for when postcss-calc is not included

[alex-page]

Co-Authored-By: Ben Scott 227292+BPScott@users.noreply.github.com

WHY are these changes introduced?

Fixes #4232

WHAT is this pull request doing?

Unitless calc was breaking when users did not provide postcss-calc as a plugin. This fixes the outline on components like <Button plain> where the output of the focusring mixin was calc(0 + 1rem).

How to 🎩

Remove this from ./storybook/main.js

https://github.com/Shopify/polaris-react/blob/9081df8bde8c682240fbc1cccba35bb5dbdd6d08/.storybook/main.js#L57-L63

Then open the plain button example.

🎩 checklist

• Tested on mobile
• Tested on multiple browsers
• Tested for accessibility
• Updated the component's README.md with documentation changes
• Tophatted documentation changes in the style guide
• For visual design changes, pinged one of @ HYPD, @ mirualves, @ sarahill, or @ ry5n to update the Polaris UI kit

[BPScott]

Turns out calc(0 + 1rem) is error in browsers. However postcss-calc is totally fine with simplifying it down to calc(1rem).

This PR includes an update to @shopify/postcss-plugin which we thought might be a fix along the way. Turns out it wasn't the root cause but hey we might as well keep it anyway.

[github-actions]

size-limit report

Path	Size
cjs	141.86 KB (0%)
esm	95.43 KB (0%)
esnext	138.62 KB (+0.04% 🔺)
css	33.72 KB (+0.12% 🔺)

[kyledurand]

Code looks good, did a quick 🎩 as well just in case