Add enrichR to R-Universe

[dcollins15]

This PR adds the enrichR package to satijalab.r-universe.dev in an effort to address the following NOTE currently being thrown by R CMD check for Seurat:

* checking CRAN incoming feasibility ... [31s] NOTE
Maintainer: 'Rahul Satija <[email protected]>'

Suggests or Enhances not in mainstream repositories:
  BPCells, enrichR, presto
Availability using Additional_repositories specification:
  BPCells   yes   https://bnprks.r-universe.dev   
  presto    yes   https://satijalab.r-universe.dev
  enrichR    no   ?                               
  ?           ?   https://cran.r-universe.dev  

In satijalab/seurat#9565 we tried to address this issue by adding the cran.r-universe.dev remote—enrichR has since been dropped from this too 😢

Unless I'm missing some downside, adding the package to our universe seems like the easiest/best way to unblock satijalab/seurat#9567. Once enrichR makes it back onto CRAN, we can drop it from the packages.json and things should go back to business as usual, with no updates to Seurat needed 👌