Add diffThreshold to storybook (#8778)

This is still a test but I think needs to be merged into main before we
can compare against it. I will follow up with a test PR to test this.
This new threshold will catch smaller changes like 4px border-radius
changes that weren't caught in [this
pr](#8626)

I compared the two snapshots using the chromatic tool
[here](https://6262c53f521620003ac2ff49-ukmsdlppcb.chromatic.com/?path=/story/stories-diff-threshold-check--test-yours-out)


https://user-images.githubusercontent.com/6844391/228541322-b5fddce0-10cb-4b41-bfda-8873ee6651b0.mp4

.changeset/slimy-comics-cough.md

@@ -0,0 +1,5 @@
+---
+'@shopify/polaris': patch
+---
+
+Increased precision of chromatic snapshot diffs

polaris-react/.storybook/preview.js

@@ -90,7 +90,11 @@ const viewPorts = Object.entries({
   };
 });
 
-export const parameters = {viewport: {viewports: {...viewPorts}}};
+export const parameters = {
+  viewport: {viewports: {...viewPorts}},
+  // Increases precision of rendered snapshot diffs. Default is 0.063
+  chromatic: {diffThreshold: 0.03},
+};
 
 export const decorators = [
   GridOverlayDecorator,