Sync: Add enable_instant_search to syncing for Search

### Impacted plugin

Jetpack

### Steps to Reproduce

1. Make sure enable_instant_search is not set on your site.
2. Buy a Search plan (which will remotely set enable_instant_search)
3. Look at the option on WP.com cache site

### A clear and concise description of what you expected to happen.

The enable_instant_search option on WP.com should be set to the same value as what is on the Jetpack site.

### What actually happened

We aren't setting/syncing the option. See p2-panfyZ-R9#comment-2031

We're using this option to help control how much content we index, so it is pretty important that it gets synced correctly.

### Other information

I don't see the option in any of the Sync modules: https://github.com/Automattic/jetpack/search?q=enable_instant_search

So I think we need to just add it to the Sync list. It may be sufficient to just add it to https://github.com/Automattic/jetpack/blob/trunk/projects/packages/sync/src/modules/class-search.php

### Reproducibility

Consistent

### Severity

Most (> 50%)

### Operating System

_No response_

### OS Version

_No response_

### Browser

Other / Not applicable

### Browser Version(s)

_No response_

Start in projects/packages/sync/src/modules/class-search.php, where the Search sync module is named in the issue, and inspect how existing options are added to the sync list. Reproduce the Jetpack-to-WP.com cache-site scenario, then verify that enable_instant_search matches the Jetpack site's value after syncing.

Written by the indexing model from the issue text.