picom: sync module with the NixOS version

This brings a few advantages:

- Use of float instead of strings to represent float values,

- Use of structure settings, and

- Better type checking for some settings

Also add thiagokokada as codeowner of picom.

.github/CODEOWNERS

@@ -386,6 +386,9 @@
 
 /modules/services/pasystray.nix                       @pltanton
 
+/modules/services/picom.nix                           @thiagokokada
+/tests/modules/services/picom                         @thiagokokada
+
 /modules/services/pbgopy.nix                          @ivarwithoutbones
 /tests/modules/services/pbgopy                        @ivarwithoutbones