feat: add strategy for R packages

[jolars]

Fixes #2151 🦕

I'm new to typescript and javascript, so please take this as a work in progress.

One thing that I couldn't figure out how to handle was how to create a NEWS.md file by default, without explicitly having to specify --changelog-path=NEWS.md. It would be nice if this could be a per-strategy default, but maybe I'm missing something obvious and otherwise I guess that should go into its own issue.

Another issue is that typically R packages have a NEWS.md file with a somewhat different structure, and it doesn't expect a top-level header but instead a package version (typically including name, but I don't think that's necessary). Otherwise some of the tools used in the R community, such as pkgdown have a hard time dealing with the changelogs. A typical NEWS.md file could look like this:

# foofy 1.0.0

## Major changes

* Can now work with all grooveable grobbles!

## Minor improvements and bug fixes

* Printing scrobbles no longer errors (@githubusername, #100).

* Wibbles are now 55% less jibbly (#200).

It was straightforward to remove the Changelog header from the generation. Maybe the header levels should be decremented now, but I'm not sure it matters.