chore: add internal to export paths (#491)

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**Which problem is this pull request solving?**

Adding `@netlify/functions/internal` to export paths

**List other issues or pull requests related to this problem**

Follow-up to changes in #473

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Author: mrstork

package.json

@@ -7,6 +7,10 @@
     ".": {
       "import": "./dist/main.mjs",
       "require": "./dist/main.js"
+    },
+    "./internal": {
+      "import": "./dist/internal.mjs",
+      "require": "./dist/internal.js"
     }
   },
   "version": "2.6.0",