Combine monitor.check and monitor.notify into a single function

Currently we use a combination of `monitor.check` and `monitor.notify` to create an alert using checks and notification rules. This is confusing for the base case user that wants to be immediately notified when an issue happens. We should create a flux function called `monitor.alert` that is the combination of both `monitor.check` and `monitor.notify`.

Link https://github.com/influxdata/influxdb/issues/15839

Start by reviewing the existing monitor.check and monitor.notify functions and the linked issue 15839 to understand their current behavior and scope. Define monitor.alert as the combined entry point, then verify that it supports the immediate-notification use case while preserving the existing checks and notification rules.

Written by the indexing model from the issue text.