Add OnNextPrompt trigger

[chrimc62]

Currently for prompts generation builds an OnEndOfActions trigger with a condition that has has !$ of the property that is in the Ask as ExpectedProperties. To make this clearer we could add a new trigger OnNextPrompt.  It would require an Ask as an action and would mine the Ask.ExpectedProperties for the ! properties and the LG template to find the required properties.  This is similar to what PVA does where the template missing variables affects the control flow.  The plus is that the condition is computed and derived from what people are doing. The minus is that the condition is no longer explicit. We should also pass through $askIf and add to the implicit condition via that.