Fix package.json exports for Node.js >= 17

We hadn’t realised that support for “subpath exports” with trailing slashes (not asterisks) was deprecated in Node.js 16 and removed in Node.js 17 https://nodejs.org/docs/latest-v18.x/api/deprecations.html#dep0148-folder-mappings-in-exports-trailing-

But fully switching to “subpath patterns” requires Node.js 12.20+ so both flavours will be needed for backwards compatibility

Fixes: #3755

package.json

@@ -12,7 +12,9 @@
       "require": "./govuk/all.js"
     },
     "./govuk/": "./govuk/",
+    "./govuk/*": "./govuk/*",
     "./govuk-esm/": "./govuk-esm/",
+    "./govuk-esm/*": "./govuk-esm/*",
     "./package.json": "./package.json"
   },
   "sideEffects": [