Xwidget and other modules

I regularly need a clean Emacs build to test stuff and compare to behaviours with other Emacs branches or forks, it would be nice to have xwidgets for example, see #13.

I can make such a PR if there is any interest on this @muep . Same applies to flags as `--with-modules`, `--with-jansson` (required for new json native parser in Emacs 27), `harfbuzz` (Also introduced in Emacs 27), `--with-cairo`, `--with-gtk3`, etc. Which are some of the most popular flags.

Of course each of these features would be on its own PR and priority would be for xwidgets and the features added in emacs 27.

Start by reviewing issue #13 and the existing Flatpak build configuration for the Emacs package, then compare how xwidgets and the listed Emacs 27 flags are represented. Done means agreeing on one feature, prioritizing xwidgets, and defining a separate PR scope; this issue names no file or test.

Written by the indexing model from the issue text.