lib: add generator for KDL

Added a generator for the KDL document language.
This is in order for home-manager to natively generate
the new config format for zellij, as described in nix-community#3364.

There is not a one to one mapping between KDL and nix types,
but attrset translation is heavily based on KDLs JSON-IN-KDL microsyntax.
The exception here is the `_args` and `_props` arguments, which lets you
specify arguments and properties as described in the spec.

See more here:
- https://kdl.dev/
- https://github.com/kdl-org/kdl/blob/main/SPEC.md

The generator also conforms to the interface from the nixpkgs manual:
https://nixos.org/manual/nixpkgs/stable/#sec-generators

Co-authored-by: Gaetan Lepage <gaetan@glepage.com>

.github/CODEOWNERS

@@ -16,6 +16,9 @@ Makefile                                              @thiagokokada
 
 /modules/launchd                                      @midchildan
 
+/modules/lib/generators.nix                           @h7x4
+/test/lib/generators                                  @h7x4
+
 /modules/misc/dconf.nix                               @rycee
 
 /modules/misc/editorconfig.nix                        @loicreynier