Add Flatpak support

[paulimer]

Hi, When Chrome or Chromium is install via Flatpak, I haven't found a way to make chromote (in my usecase, for shinytest2) use it. I have tried to set CHROMOTE_CHROME to org.chromium.Chromium, or to usr/bin/flatpak run org.chromium.Chromium, without success. I'm not a very experienced Flatpak user by any means, but would there be a way to support it? Thanks in advance