[5.x] Include Algolia highlights and snippets

[jacksleight]

This PR adds Algolia's highlights and snippets to the result data. Highlights are usually included by default, snippets can be included with these settings:

'default' => [
    'driver' => 'algolia',
    'settings' => [
        'attributesToSnippet' => ['content:40'],
        'highlightPreTag' => '<mark>',
        'highlightPostTag' => '</mark>',
    ],
],

You can then use them in results:

<a href="{{ $url }}">
    <h2>{{ search_highlights:title:value }}</h2>
    <p>{{ search_snippets:content:value }}</p>
</a>

The key names are based on the convention established in the comb driver for snippets.

[daun]

@jacksleight I just discovered this – great stuff! Got me inspired to implement proper snippet generation for the Loupe search driver. While Loupe already includes highlighting, it doesn't currently offer snippets, so this seemed like a nice feature to add. Thanks for the inspiration!