feat(to_additive): option to not translate operations on a type (#19297)

* Also some cleanup in the `ToAdditive/Frontend` file (please review the first 3 commits separately), only the second commit actually changes the functionality of `to_additive`. The main cleanups: move doc-strings to the syntax declarations, remove the module doc which was just duplicating the `to_additive`-doc, and let a few internal functions depend on `Environment` instead of multiple other arguments.
* In #19687 this is done for `Monoid.End` (in a separate PR, since that requires some library fixes).