dillon1000/react

Parallelize CircleCI jobs using workflows (#15704)

Updates the CircleCI config to use the workflows features to run jobs in
parallel, instead of the `parallelism` option. This change alone doesn't
improve the overall build time much, since almost all of the total time
is spent running the Rollup script, which runs entirely sequentially.
But it does improve reporting, and should make it easier to add
additional parallel jobs in the future.