The picom module's `services.picom.settings` converts Nix expressions
to the libconfig format. However, that option currently does not have
a way to specify libconfig lists (enclosed in parentheses `()`), and
can only specify arrays (enclosed in square brackets `[]`). This makes
it impossible to specify certain picom configuration options, such as
animations (which are specified using lists).
This commit adds an `extraConfig` option to the picom module as an
escape hatch so that users can still specify picom options that are not
currently representable through `services.picom.settings`.
0bc5f414f8