Add a check for upgrader_process_complete

## What problem would the enhancement address for VIP?

Because the contents of `/wp-content/plugins` and `/wp-content/themes` are setup as `read-only` directories, and end-users can't install or update plugins from within the `wp-admin`, the [hook `upgrader_process_complete`](https://developer.wordpress.org/reference/hooks/upgrader_process_complete/) will never run for either plugin install or update on the VIP Go platform.

When a VIP customer installs a plugin (via the [GitHub repo](https://docs.wpvip.com/technical-references/vip-codebase/)) with a function tied to that hook, their development team has to reverse engineer the plugin's code to execute that functionality.

This is not entirely unique to VIP, but only impacts WordPress environments where the `/wp-content/plugins` and `/wp-content/themes` are `read-only`, so it may not be a good fit for the [WordPress Coding Standards](https://github.com/WordPress-Coding-Standards/WordPress-Coding-Standards/).

## Describe the solution you'd like

A sniff that will flag places where the [`upgrader_process_complete` hook](https://developer.wordpress.org/reference/hooks/upgrader_process_complete/) is used. Preferably as an error because it is functionality that will not work.

## What code should be reported as a violation?

```
add_action( 'upgrader_process_complete', 'my_upgrade_function',10, 2);
```

## What code should *not* be reported as a violation?

I'm unaware of cases where this might be detected as a false positive.

## Additional context

_None applicable_

Inizia leggendo le convenzioni esistenti del repository per sniff e test, quindi segui come vengono rilevate le chiamate all’hook WordPress upgrader_process_complete. L’issue fornisce l’hook target e un esempio di add_action che viola la regola; il lavoro è completo quando lo sniff segnala quell’utilizzo come errore senza che sia stato identificato alcun caso di falso positivo.

Scritto dal modello di indicizzazione a partire dal testo della issue.