Change RStudio contributions to BSD

The RStudio contributions should be licensed under the same license as the rest of Ace, BSD. These were [merged into Ace](https://github.com/ajaxorg/ace/pull/1117) a while ago, but I think they were copy-pasted before Ace change its license.

I'll follow up with RStudio devs to make sure this is a desired change. Changing the license benefits the community; otherwise, many projects run the risk of not bundling Ace support for these highlighter modes.