Add an icon for R files (.R/.r)

## Problem
ElementaryOS currently do not have a icon for files with the .R or .r extensions, which are associated with the largely used R language.

## Proposal
Add a icon for R files. One simple option would be to put the [logo](https://www.r-project.org/logo/Rlogo.svg) over the blank "paper" icon.


Here's the [license for the official R logo](https://www.r-project.org/logo/):

```
The CC-BY-SA 4.0 license allows you to share and adapt the logo for any purpose, including commercial use, provided that you give appropriate credit, provide a link to the license, and indicate if any changes were made. It is also one-way compatible with the GNU General Public License version 3 (GPL‑3). However, CC-BY-SA 4.0 is not compatible with GPL‑2. If you wish to incorporate the logo into works licensed under GPL-2 then you must use the GPL-2 license for the logo.

Note that special considerations apply when using the GPL-2 license for the logo or when incorporating it into works licensed under GPL-3 . In particular, the GPL includes the concept of source, which is defined as “the preferred form of the work for making modifications to it”. We consider the vector version of the R logo in SVG format to be the source by this definition.
```

How would that work? Where would we put the link to the license?

If you guy are ok with this, I can work on a PR.

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