Is there an R online compiler that enables to run code line by line?

Some Jupyter variants let you run a single line or a selection, Google Colab ( https://colab.to/r for new R Notebook ) or Kaggle ( https://www.kaggle.com/docs/notebooks ) for example, both use Ctrl+Shift+Enter

Bit different take would perhaps be RStudio in Binder.
Example - http://mybinder.org/v2/gh/binder-examples/r/master?urlpath=rstudio
More details - https://github.com/binder-examples/r

Have you tried RStudio cloud? https://rstudio.cloud/