- unify `map!` statements when possible
- rename `+vertico--embark-target-package!` to
`+vertico--embark-target-package` and autoload it
- set `completion-in-region-function` to a wrapper function instead of
changing it with a hook
- use `:override` advice instead of `fset` for `multi-occur`
- document what `vertico-directory-tidy` does
- move `:init` contents to `:config` when possible
- Rename module from `:completion selectrum` to `:completion vertico`
- Rename all files involved
- Do *not* yet rename all the functions, as that messes up git's rename
detection.
