Prepare for bbr.bayes package

[kyleam]

The plan is to split the bbr_stan_alpha functionality (#358) into its own packages. That, however, requires some changes on bbr's side. This PR uses bbr_stan_alpha as the base (and includes all the commits in that PR ~except for the last one, which is just a DESCRIPTION change~). The commits on top of bbr_stan_alpha remove the parts that are headed to the dedicated package, and add and expose functionality for that package to hook into (e.g., new generics).

Note that, in addition to "separate package support" changes, this PR would bring in the submit_mode(..., .overwrite = ...) change from the bbr_stan_alpha branch (mostly from commit 1c44d1c). This came in separately in https://github.com/metrumresearchgroup/bbr/pull/547

I'm marking this as a draft because see March 9 update below

• the plan it to hold of merging this until after the upcoming release

• I've mostly been developing this with automated tests. It'd be good to test out some of the bbr_stan_alpha based demos that are being worked on internally. (Edit: Played around with @seth127's model-management-bbr-bayes.Rmd. Need to add a library(bbr.bayes) call of course, but didn't run into any issues.)

• The base is bbr_stan_alpha right now, since that's probably the most helpful diff, but it could be changed to main before the merge.

  The diff with main is of course also useful. Here it is: https://github.com/metrumresearchgroup/bbr/compare/main...bbr-bayes-split

  Update: I've changed the base to main because that's 1) the eventual merge target and 2) the branch that is likely to continue to be merged into this one to resolve conflicts (and so it's useful for this PR to not show commits for main's side). The bbr_stan_alpha diff is https://github.com/metrumresearchgroup/bbr/compare/bbr_stan_alpha...bbr-bayes-split

cc @seth127

Update 2023-03-09: Anticipating a bbr.bayes release in the next 1-2 months, so this will be reviewed, merged, and released in the near future.

[kyleam]

Merged in main to resolve conflicts with 1.5.0 release. Left DESCRIPTION version at 1.5.0, but we can bump to dev version and tag whenever we need a new dev release for bbr.bayes.

[kyleam]

Forge ahead!

All right, in it goes. Thanks for the review.