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>

tests/lib/generators/default.nix

@@ -0,0 +1 @@
+{ generators-tokdl = ./tokdl.nix; }

tests/lib/generators/tokdl-result.txt

@@ -0,0 +1,41 @@
+a 1
+b "string"
+bigFlatItems 23847590283751 1.239000 "multiline \" \" \"\nstring\n" null
+c "multiline string\nwith special characters:\n\t \n \\" \"\n"
+extraAttrs 2 true arg1=1 arg2=false {
+	nested {
+		a 1
+		b null
+	}
+}
+flatItems 1 2 "asdf" true null
+listInAttrsInList {
+	list1 {
+		- {
+			a 1
+		}
+		- {
+			b true
+		}
+		- {
+			c null
+			d {
+				- {
+					e "asdfadfasdfasdf"
+				}
+			}
+		}
+	}
+	list2 {
+		- {
+			a 8
+		}
+	}
+}
+nested {
+	- 1 2
+	- true false
+	- 
+	- null
+}
+unsafeString " \" \n 	 "

tests/lib/generators/tokdl.nix

@@ -0,0 +1,53 @@
+{ config, lib, ... }:
+
+{
+  home.file."result.txt".text = lib.hm.generators.toKDL { } {
+    a = 1;
+    b = "string";
+    c = ''
+      multiline string
+      with special characters:
+      \t \n \" "
+    '';
+    unsafeString = " \" \n 	 ";
+    flatItems = [ 1 2 "asdf" true null ];
+    bigFlatItems = [
+      23847590283751
+      1.239
+      ''
+        multiline " " "
+        string
+      ''
+      null
+    ];
+    nested = [ [ 1 2 ] [ true false ] [ ] [ null ] ];
+    extraAttrs = {
+      _args = [ 2 true ];
+      _props = {
+        arg1 = 1;
+        arg2 = false;
+      };
+      nested = {
+        a = 1;
+        b = null;
+      };
+    };
+    listInAttrsInList = {
+      list1 = [
+        { a = 1; }
+        { b = true; }
+        {
+          c = null;
+          d = [{ e = "asdfadfasdfasdf"; }];
+        }
+      ];
+      list2 = [{ a = 8; }];
+    };
+  };
+
+  nmt.script = ''
+    assertFileContent \
+      home-files/result.txt \
+      ${./tokdl-result.txt}
+  '';
+}