Make invisible result warning opt-in

[nischalshrestha]

Fixes #369

Allow the invisible result warning to be configured/disabled as an exercise chunk option called "warn_invisible". The default behavior is no warning.

Example file

---
title: "Warn invisible"
output: learnr::tutorial
runtime: shiny_prerendered
---

The following code chunks demonstrates normal exercise without a `warn_invisible` option set.

```{r no-invisible-warning, exercise=TRUE}
x <- 1
```

This chunk sets a `warn_invisible` option to `FALSE`.

```{r with-invisible-warning-1, exercise=TRUE, exercise.warn_invisible = FALSE}
x <- 1
```

This chunk sets a `warn_invisible` option to `TRUE`.
```{r with-invisible-warning-2, exercise=TRUE, exercise.warn_invisible = TRUE}
x <- 1
```

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1 out of 2 committers have signed the CLA.

✅ nischalshrestha
❌ Nischal Shrestha

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[ericemc3]

I am interested by this feature

[Brunox13]

Looks like this valuable PR is hung up on a likely erroneous CLA signature requirement for a non-existing alias of the same user?

[cpsievert]

LGTM pending comment (and merge conflicts)