Refactor alerts.replaceChannels.js snippet to align with code sample snippet guidelines

The current version of [alerts.replaceChannels.js] contains four different region tags that demonstrate 3 (or, arguably 4) different actions. The sample flow does three different actions:

* runs through hardcoded list of notification channel names
* calls API to enable each channel using its name
* on (_any_) creates a new email notification channel with that name
* modifies hardcoded alert policy to send notifications to the hardcoded list of channels

The code should be split into 3 snippets (a snippet per file) to demonstrate the following three actions:

1. Update notification channel. The snippet can be conditioned that the channel should exist.
1. Create a new notification channel.
1. Update an alert policy with a notification channel.

All snippets should provide a reasonable error handling. Inputs should be clearly commented to explain expectations. Error handling should, at least, handle errors when inputs are invalid.

[alerts.replaceChannels.js]: https://github.com/GoogleCloudPlatform/nodejs-docs-samples/blob/main/monitoring/snippets/alerts.replaceChannels.js