Workflow to choose between RAW and JPG

The first issue, is that both the Raw and the JPG will need to be shown. This is controlled (for future imports going forward), by a checkbox setting "Treat JPG files next to Raw files as separate photos" in LR Preferences.

Getting existing imported Raw + JPG pairs which currently show up as only one image, to show up as two, will require some extra steps.

By sorting the images by Capture Time, you will ensure that these two versions of a given exposure are seen alongside each other.

browse through the pictures and decide for each one to keep the RAW or the JPG. If choosing the RAW the JPG should be deleted and the other way round

If you restate that another way: "for each one, to DELETE either the Raw or the JPG", then that's the task simplified into one action.

You can mark images as provisionally Rejected with the X key, check and review, and then clean out all of the currently marked rejects in a batch. Also you can Remove these images from being imported, without deleting them from disk - which allows for a future change of mind (with reimport).

I don't know of a plug-in or external program that will do that. But you can scroll through the programs and decide for yourself and press the letter X to mark images for deletion. Then when you decide that you are ready to discard them you can press Ctrl+Bkspc on the PC or the equivalent on the Mac to delete all of the images that have been marked for deletion.  The images that you marked for deletion will be grayed out in the film strip or in the grid mode in the library so that they are easily identifiable.