fix: import function template metadata as module

[cubeghost]

🎉 Thanks for submitting a pull request! 🎉

Summary

Fixes #6250
https://linear.app/netlify/issue/CT-1059/creating-new-functions-in-the-cli-via-ntl-functionscreate-is-broken

When we moved to using ESM, we left these function template metadata files (.netlify-function-template.js) alone. Some of the templates don't have package.json files, and those continued to be imported fine (including the two used in existing integration tests) because the closest package.json (in the root directory) had type: module. Templates with their own package.json files failed because those didn't have type: module. Since those template-specific package.jsons are related more to the actual {{name}} template file and not the metadata file, I decided to leave them alone and instead rename the metadata files to use .mjs across the board.

---

For us to review and ship your PR efficiently, please perform the following steps:

• Open a bug/issue before writing your code 🧑‍💻. This ensures we can discuss the changes and get feedback from everyone that should be involved. If you`re fixing a typo or something that`s on fire 🔥 (e.g. incident related), you can skip this step.
• Read the contribution guidelines 📖. This ensures your code follows our style guide and
  passes our tests.
• Update or add tests (if any source code was changed or added) 🧪
• Update or add documentation (if features were changed or added) 📝
• Make sure the status checks below are successful ✅

A picture of a cute animal (not mandatory, but encouraged)

[github-actions]

📊 Benchmark results

Comparing with f0af180

• Dependency count: 1,315 (no change)
• Package size: 295 MB ⬇️ 0.00% decrease vs. f0af180
• Number of ts-expect-error directives: 1,007 (no change)

[ndhoule]

❤️