Hook into `?` when loading

Going forward, this package is intended to be used without being attached. It therefore can’t simply shadow the definition of `base::?` by exporting it.

Instead, consider providing an appropriate `.onLoad` hook that attaches a shim environment, similar to `devtools::load_all`.

In addition, this should hook into {pkgload}’s `onLoad` and somehow patch `pkgload::insert_global_shims`, if possible without violating CRAN guidelines (note: we also need to do this for old versions of {devtools}).