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>

modules/lib/default.nix

@@ -6,6 +6,7 @@ rec {
   assertions = import ./assertions.nix { inherit lib; };
 
   booleans = import ./booleans.nix { inherit lib; };
+  generators = import ./generators.nix { inherit lib; };
   gvariant = import ./gvariant.nix { inherit lib; };
   maintainers = import ./maintainers.nix;
   strings = import ./strings.nix { inherit lib; };