Support for the R programming language

Add support for Harper in [R](https://www.r-project.org) files.

I'm not exactly sure what else would be helpful in the issue tracker. R comments are generally started with `#`. There are other specific [roxygen2](https://roxygen2.r-lib.org/index.html) comments for documentation that are started with `#'` syntax. Harper looks really cool and I'm looking forward to using it! Thanks!

Start by locating Harper’s existing language-support entry points and tests, then compare how another supported language handles # comments and documentation comments. Done means R files are recognized and checked without incorrectly flagging valid R or roxygen2 syntax.

Written by the indexing model from the issue text.