feat(exercise): Support `exercise.pipe` option

[gadenbuie]

Globally choose pipe characters, or set them per exercise. Default is chosen based on the current version of R in the rendering environment.

Supersedes #786

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You can set the characters used for the pipe shortcut at the tutorial level with

knitr::opts_chunk$set(exercise.pipe = "%>%")

Or you can set the pipe option at the individual level using the exercise.pipe chunk option.

```{r base-pipe, exercise=TRUE, exercise.pipe="|>"}
mtcars count(cyl)

By default, if not set otherwise set, learnr will use the base R pipe (`|>`) when the tutorial is rendered in R >= 4.1.0. 

[statik]

@gadenbuie thank you for this!