Reference/Query trigger status

Like switches, we should be able to reference a trigger to determine its state.

For example:
```
story.registerTrigger({
label: 'created trigger feature',
trigger: () => { /* something */ },
then: () => { /* something else */ }
```
We should be able to query the status of that trigger, like this:
```
story.triggerStatus('created trigger feature')
// Returns false
```
or
```
story.fired('created trigger feature')
// Returns false

Start by locating the implementation of story.registerTrigger and the existing switch state handling. Determine how trigger state should be represented and whether triggerStatus or fired is the intended API, then verify that querying the example trigger reports its state consistently.

Written by the indexing model from the issue text.