Make tutorials printable

[siebrenf]

see FR

PR task list:

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

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[gadenbuie]

Thank you for packaging this in a pull request, but I don't think we'd want to add a print button to every learnr tutorial and I don't think we'd want to add an argument to learnr::tutorial() to cover this feature.

Another possible solution would be to add a function — tutorial_print_button() or tutorialPrintButton()? — that would wrap the CSS and JavaScript in this PR that could be called by a user to add a print button, either to the tutorial footer or directly wherever the function is called.

That approach would also solve the problem of not wanting to add the print CSS to tutorials where authors don't want users to print the tutorial to "see ahead".

[siebrenf]

I thought I'd pack it up since you've already wrote the whole solution, but I understand this implementation might not be desired. Unfortunately, I have no idea how to implement your suggestion (3 languages in 1 function, adding elements to the footer and CSS). Feel free to close this PR if it's easier to start from scratch/too much of a hassle!