Infinite Scroll: Check for infiniteScroll when enqueuing the-neverending-homepage

Jetpack does a good job checking to see if `the-neverending-homepage` (ie `infinity.min.js`) needs to be loaded by using the [`is_last_batch()`](https://github.com/Automattic/jetpack/blob/master/modules/infinite-scroll/infinity.php#L309) function.

However, if `the-neverending-homepage` is enqueued outside of [`action_template_redirect()`](https://github.com/Automattic/jetpack/blob/master/modules/infinite-scroll/infinity.php#L424) on a page with fewer than `posts_per_page` a JavaScript error will appear:
```
ReferenceError: infiniteScroll is not defined
```

This is due to `var infiniteScroll` never being defined as it only happens when `action_template_redirect()` is true.

#### Steps to reproduce the issue

1. Enable Jetpack's Infinite Scroll module and make sure your theme has support for IS.
2. Manually enqueue `the-neverending-homepage` via a plugin or theme:
```
function my_infinite_scroll_scripts() {
wp_enqueue_script( 'the-neverending-homepage' );
}
add_action( 'wp_enqueue_scripts', 'my_infinite_scroll_scripts' );
```
3. Load the module on a page that has fewer posts than `posts_per_page`. In testing I enqueued the script on the "author" page and then visited an author with only a few post. This may also depend on `infinite_scroll_archive_supported` being `true`.

4. Note JavaScript error.

#### What I expected
No JavaScript error, just IS being loaded but having no effect.

#### What happened instead
A reference error.

#### Proposed Fixed

A quick fix would be to wrap `infinity.js` in a conditional statement like `if (typeof infiniteScroll !== 'undefined') {`. Would this be an acceptable solution? If so, I can create a PR. Otherwise, we may need to look at longer-term solutions like defining `var infiniteScroll` inside the script itself.

Start in modules/infinite-scroll/infinity.php at is_last_batch() and action_template_redirect(), then inspect the infinity.js script that references infiniteScroll. Reproduce the issue by manually enqueueing the-neverending-homepage on a page with fewer posts than posts_per_page. Done means the script can load without a ReferenceError and has no effect when infiniteScroll is unavailable.

Written by the indexing model from the issue text.