firebase init functions to set package.json's engines.node version to the dev's machine

When running `firebase init functions`, package.json setup on the dev machine [hardcodes the `engine.node` attribute](https://github.com/firebase/firebase-tools/blob/master/templates/init/functions/typescript/package.lint.json#L13):

```
"engines": { "node": "14" }
```

`engine.node` attribute is used at `firebase deploy` time to figure out the runtime of the deployed cloud function instance.

In `npm`, it's okay for node version specified in the `engines.node` to differ w/ the node version on the developer's machine, but `yarn` will [throw an error if they differ](https://classic.yarnpkg.com/en/docs/cli/install/#toc-yarn-install-ignore-scripts).

It would be nice if the `firebase init functions` can dynamically set the engine. If dev is using a node version unsupported by Google Cloud Function, we can probably error.

Relates to https://github.com/firebase/firebase-functions/issues/782

Start with templates/init/functions/typescript/package.lint.json and trace the firebase init functions flow that generates package.json. Confirm how the developer's Node version is detected and how supported Cloud Functions runtimes are represented; done means generated engines.node matches the local version when supported and yarn no longer rejects the setup.

Written by the indexing model from the issue text.