chore(blog): Yarn PnP install of the unified chain + jest transform fix

Adds remark-gfm/directive/parse/rehype, rehype-pretty-code, shiki,
unified, unist-util-visit; retires remark/remark-html. Under PnP there
are no real node_modules dirs — cache zips live under paths *containing*
/node_modules/, so jest's transformIgnorePatterns now only excludes the
css module mocks, keeping the ESM-only unified chain transformable.

jest.config.js

@@ -24,9 +24,13 @@ const customJestConfig = {
     '<rootDir>/node_modules/',
     '<rootDir>/tests/e2e/',
   ],
-  // Allow JSX in tests without explicit React import if using React 17+ JSX transform
+  // Allow JSX in tests without explicit React import if using React 17+ JSX transform.
+  // Under Yarn PnP there are no real node_modules dirs — dependencies live in
+  // cache zips whose paths *contain* /node_modules/ segments, so excluding that
+  // pattern would skip transforming the ESM-only unified chain. Only the css
+  // module mocks (handled by next/jest) stay out of the transform.
   transformIgnorePatterns: [
-    '/node_modules/',
+    '^.+\\.module\\.(css|sass|scss)$',
   ],
   // Make sure Jest can find our mocks
   moduleDirectories: ['node_modules', '<rootDir>'],