Fix package.json exports for Node.js <= 12.18

We switched to “subpath patterns” recently (supported in Node.js 12.20+) but hadn’t realised that “subpath exports” with trailing slashes (not asterisks) were still needed for older versions

We need both flavours because trailing slashes in exports were deprecated in Node.js 16
https://nodejs.org/docs/latest-v18.x/api/deprecations.html#dep0148-folder-mappings-in-exports-trailing-

Fixes: #3755

packages/govuk-frontend/package.json

@@ -17,7 +17,10 @@
       "import": "./dist/govuk-esm/all.mjs",
       "require": "./dist/govuk/all.js"
     },
-    "./*": "./*"
+    "./*": "./*",
+    "./dist/": "./dist/",
+    "./govuk-prototype-kit.config.json": "./govuk-prototype-kit.config.json",
+    "./package.json": "./package.json"
   },
   "sideEffects": [
     "**/vendor/**"