Feature request: Add support for R-universe in environment.yaml similar to support for pip

### Checklist

- [x] I added a descriptive title
- [x] I searched open requests and couldn't find a duplicate

### What is the idea?

[R-universe](https://r-universe.dev/) is a CRAN compatible repository for R packages created in [2021](https://ropensci.org/commcalls/may2021-r-universe/) that [graduated to a R Consortium top level project](https://r-consortium.org/posts/r-universe-named-r-consortiums-newest-top-level-project/). R-universe has packages not published on CRAN or conda-forge that are valuable to R users.

Also, [R-universe builds WASM binaries](https://ropensci.org/blog/2023/11/17/runiverse-wasm/) that can be valuable for projects like JupyterLite.

### Why is this needed?

Like the native support for PyPI expand the Python user base, the native support for R-universe will expand the R user base and reduce the burden when using R packages that are not already packaged by conda-forge.

### What should happen?

- [ ] [Conda Enhancement Proposal](https://github.com/conda/ceps) create and submit
- [ ] Conda Enhancement Proposal accepted / approved
- [ ] Implementation by clients on their own discretion

### Additional Context

From [CEP 24 - Specification of environment.yml input files](https://github.com/conda/ceps/blob/main/cep-0024.md):

> `dependencies`
>
> [...]
>
> This section can also contain "subsection" dictionaries [...]
>
> Currently known subsections include pip [...]
>
> Additional subsections are allowed. [...]

The feature request https://github.com/conda/conda/issues/10708 was "automatically marked as stale".