Implement a trigger for core.SystemStartlevelTrigger

[jimtng]

https://github.com/openhab/openhab-core/blob/main/bundles/org.openhab.core.automation/src/main/java/org/openhab/core/automation/internal/module/handler/SystemTriggerHandler.java

Perhaps we could modify on_start to have a start_level: argument?

[boc-tothefuture]

That was my thinking too, an optional either symbol or number.