Add event handler system

[wch]

This adds a general-purpose event handler system. Closes #357.

It adds:

• event_register_handler()
• one_time()

It also converts many of the existing event-like functions into to true event handlers.

It adds a new event, "section_viewed", which is triggered every time a section becomes visible.

The event_register_handler() function can be used to execute code every time an event occurs. For example, this will print a message to the console every time any section becomes visible.

```{r context="server-start"}
event_register_handler("section_viewed", function(session, event, data) {
  message("event: section_viewed: ", data$sectionId)
})
```

The one_time() function can be used to only execute code the first time an event happens AND some condition is satisfied. This can be used to execute code the only the first time a section is viewed:

```{r context="server-start"}
event_register_handler("section_viewed",
  function(session, event, data) {
    one_time(
      session,
      cond = data$sectionId == "section-exercise-with-hint",
      expr = { message("Seeing ", data$sectionId, " for the first time.") }
    )
  }
)
```

---

Usage notes

If a section doesn't have a name, then one will be auto-generated. In the example below, the first one will trigger a section_viewed event where data$sectionId is "section-section" and the second will be "section-section-1".

###

foo

###

bar

You can also add specific section IDs without having any text show up. This is better in practice, so that adding a section in the middle doesn't alter up later section IDs that you're looking for. In the example below, the first one will be "section-first" and the second will be "section-second".

### {#first}

foo

### A title here {#second}

bar

---

Implementation notes

The events starts in tutorial-format.js. There are shown and hidden events for the topics and sections. Unfortunately, they fire too often, and sometimes (for example) a shown event occurs when a section isn't actually visible. The code in that file converts those trigger-happy events into state, in the form a Shiny input value. It sets input$`tutorial-visible-sections` to a character vector that contains the IDs of the visible sections.

On the R side, the states from input$`tutorial-visible-sections` are converted back to events, using an observer which calls event_trigger("section_viewed", ....).

[schloerke]

Caching within package is ok. We do this for global setup chunk and exercise chunks.

Let's add a "reset event handlers cache" when starting a learnr tutorial in tutorial().

Let's also reset the global setup chunk and exercise chunk cache when resetting the registered event handlers.