Registers brms_fit and brms_prior_fit built-in executors. The
loo, compare, ppc, loo_pit, and sbc kinds are shared with
bg_use_cmdstanr() and are registered here too so a brms-only project can
use them without calling both setup functions. A neutral data kind (an
alias of the stan_data executor) is registered as well, so brms-backed
graphs need not label plain data frames with a Stan-specific kind.
Details
The brms fit executors pass backend and algorithm to brms::brm()
explicitly, from node params or else the brms.backend and
brms.algorithm options resolved at execution time. Their job records'
provenance (see bg_node_pad()) carries the family, the applied priors,
and the sampler settings read back from the fit.