Make 'continue' button customisable in sub-topics

[dave-mills]

Customise 'continue' button text

This PR adds the ability to specify custom 'continue' button text for each subtopic with a data-continue-text attribute. It uses the same syntax as for adding the progressive reveal option to individual topics:

### Subtopic Heading {data-continue-text: "Show Next Section"}

Any subtopics without this attribute will still use the default "Continue" text, as defined by the translation files.

Rationale

We use learnr a lot for writing interactive modules for online or mixed-learning R, statistics and data management courses. Often, we use progressively revealed subtopics for self-assessed quizzes, and we wanted a way to replace the 'Continue' text with e.g. "Show Solution", or "Next Question". I made a hack-y version for us with a custom JavaScript file, but I thought it'd be nicer to have a built-in option.

Concerns

I understand that this may be a very niche feature, so you may not want to include it for that reason. I also realise that the custom text is not translatable with this setup, but given that each document only uses 1 language at a time right now, I figured that making the custom text translatable wasn't vital. But if/when learnr is updated to allow multiple languages at the same time this would need fixing.

PR task list:

• Update NEWS
• Add tests (if possible)
• Update documentation with devtools::document()

(btw - I didn't know if/how this could be testable. I'm not very familiar with R package development, so happy to add tests if needed or tidy up other package-dev things!)

Thanks for your time, and thanks for such a great package for building training resources!

[CLAassistant]

All committers have signed the CLA.

[dave-mills]

Example of an Rmd file with this feature:

---
title: "Example custom button text"
output:
  learnr::tutorial:
    progressive: yes
runtime: shiny_prerendered
description: Example learnr tutorial with custom continue buttons
---

```{r setup, include=FALSE}
library(learnr)
```

## Topic 1

### Question 1 {data-continue-text="Show Solution"}

Question text goes here.

### Solution 1 {data-continue-text="Next Question"}

Solution text goes here.

### Question 2 {data-continue-text="Show Solution"}

Question 2 text.

### Solution 2

The next button has the 'default' text for your langauge.

## Topic 2

etc...

[gadenbuie]

Thanks! I refactored the code slightly and added a manual test. Overall this looks great! It'd be nice to have this documented somewhere in the learnr website, but I can add that later before the next CRAN release.