Decker's font editor is a deck made in Decker itself, rather than a built-in feature.
If you download Decker on itch, inside the zip file is a directory named "examples", and inside that directory is a deck named "fontedit.deck".
If you run Decker and then open "fontedit.deck", you will have an editor like the screenshot above. As I noted earlier, if you make changes to this deck and save it, you can then import your newly created/modified fonts from fontedit.deck into other decks.
Is that more clear?