Add CODEOWNERS (#10725)

### WHY are these changes introduced?

Major version work is moving quickly. We need visibility into changes
shipping to main to avoid reverts and conflicts and divert changes to
`next` when needed.

.github/CODEOWNERS

@@ -0,0 +1,20 @@
+# Default
+* @Shopify/polaris-team
+
+# React, TypeScript, JavaScript
+*.tsx @Shopify/polaris-eng
+*.ts @Shopify/polaris-eng
+*.js @Shopify/polaris-eng
+
+# Config files
+*.yml @Shopify/polaris-eng
+*.json @Shopify/polaris-eng
+
+# Documentation
+*.mdx @Shopify/polaris-team
+*.md @Shopify/polaris-team
+
+/polaris.shopify.com/content/design @Shopify/polaris-ux
+/polaris.shopify.com/content/foundations @Shopify/polaris-ux
+/polaris.shopify.com/content/patterns @Shopify/polaris-ux
+