Set tab size in Ace editor to 2

[rossellhayes]

The Ace editor used in learnr tutorials defaults to using 4 spaces for tabs, but Tidyverse style recommends two spaces.

@gadenbuie I think I made the needed change in tutorial.js, but I'm not sure how build.js works, could you help me out with that part?

[gadenbuie]

Looks good, thanks! To update the learnr javascript resources you can run the following in the Terminal in the learnr repo:

# install the required js libraries (call once)
npm install

# re-build learnr's js sources
npm run build