Make command-not-found aware of Flatpaks

## Problem

If Inkscape is not installed—or is installed as a Flatpak—and I type, `inkscape` in the Terminal, I'm told it's not found but that I can install it from the Ubuntu repos or the Snap. There is no indication that it is even available as a Flatpak, and no indication if it's actually installed as a Flatpak.

## Proposal

Instead, it would be handy for command-not-found to be aware that a potential/partial match exists as a Flatpak and offer to run that. Or if Inkscape _isn't_ installed, it would be great to get a Flatpak suggestion in there alongside the Snap and repo suggestions to install it.

## Prior Art

`flatpak install` supports partial/potential matches, i.e. `flatpak install inkscape` will correctly offer to install `org.inkscape.Inkscape` from an available remote.