[stylelint-polaris] Disallow component overrides using selector-disallowed-list (#8467)

### WHY are these changes introduced?

Fixes #7850

### WHAT is this pull request doing?

This PR configures the `selector-disallowed-list` rule in
`@shopify/stylelint-polaris` to disallow Polaris React component style
overrides via classname. This will make the tracking of overrides much
more performant than manually finding and counting them as we do
currently.

<img width="844" alt="Screenshot 2023-02-22 at 5 37 31 PM"
src="https://user-images.githubusercontent.com/18447883/220776076-f558a3cd-8a31-4c2b-95a3-b51fa9f813b9.png">

### 🎩 checklist

- [x] Tested in polaris-react SCSS files
- [ ] Added rule documentation 
- [ ] Tested in Polaris Coverage (wip)

---------

Co-authored-by: Sam Rose <sam.rose@shopify.com>
Co-authored-by: Aaron Casanova <32409546+aaronccasanova@users.noreply.github.com>

Author: 3 people

.changeset/thin-tools-sort.md

@@ -0,0 +1,5 @@
+---
+'@shopify/stylelint-polaris': major
+---
+
+Configured `selector-disallowed-list` to disallow Polaris component class name overrides

stylelint-polaris/index.js

@@ -371,6 +371,13 @@ const stylelintPolarisCoverageOptions = {
     },
   ],
   conventions: {
+    'selector-disallowed-list': [
+      [/class[*^~]?='Polaris-[a-z_-]+'/gi],
+      {
+        message:
+          'Overriding Polaris styles is disallowed. Please consider contributing instead',
+      },
+    ],
     'polaris/custom-property-allowed-list': {
       // Allows definition of custom properties not prefixed with `--p-`, `--pc-`, or `--polaris-version-`
       allowedProperties: [/--(?!(p|pc|polaris-version)-).+/],