feat(reveal.js): add support for fragment indices in line highlighting

[mcanouil]

Introduce support for fragment indices to enhance line highlighting functionality, allowing for more precise control over highlighted code segments in Reveal.js.

This adds a code-line-fragment-indices attribute to offer control over fragment.
When set, the same number of fragment indices as there are steps are required.

I'm not sure of the other changes that this new feature should be coming with.

---
format: revealjs
---

:::: {.fragment fragment-index="3"}
Step 3
:::

:::: {.columns}

:::: {.column width="50%"}
```{.r code-line-numbers="|2|3" code-line-fragment-indices="1,2,4"}
par(mfrow = c(1, 2))
plot(cars)
plot(cars, col = "red")
par(mfrow = c(1, 1))
```
:::

:::: {.column width="50%"}
```{.r code-line-numbers="|2|3" code-line-fragment-indices="1,2,4"}
par(mfrow = c(1, 2))
plot(cars)
plot(cars, col = "red")
par(mfrow = c(1, 1))
```
:::

:::

:::: {.fragment fragment-index="1"}
First step
:::

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[cderv]

Thanks for initiating !

Could be also related to

• Feature request: fragment-index for code-line-numbers #7378
• code-line-numbers and fragments not working as expected/hoped in revealjs #11585 (maybe ?)

Also, do we have an issue about this new feature with a discussion or only the Slack question?
Discussing the issue, the design is sometimes easier, and I may have missed. So maybe the one above covers it.

And regarding naming, I think I would like to handle this one too before adding new options

• Clearer argument name for Reveal.js highlighting with code-line-numbers  #4899

Just listing all this as thoughts for later.

[mcanouil]

Thanks for linking. The changes would indeed fix both issues. (I'm not linking them formally at this point to avoid risking closing the issues)

The PR is mostly here to not lose the code.

[mcanouil]

I updated the PR for it to be more complete: changelog, test, etc.