lib: Improve KDL generator (#7429)

This adds support for generating ordered children and nodes with
attributes and/or properties but no children. These are both needed to
generate zellij keybinding configuration.

tests/lib/generators/tokdl-result.txt

@@ -1,7 +1,12 @@
 a 1
+argsAndProps 1 2 a=3
 b "string"
 bigFlatItems 23847590283751 1.239000 "multiline \" \" \"\nstring\n" null
 c "multiline string\nwith special characters:\n\t \n \\" \"\n"
+duplicateChildren {
+	child 2
+	child 1
+}
 extraAttrs 2 true arg1=1 arg2=false {
 	nested {
 		a 1

tests/lib/generators/tokdl.nix

@@ -38,6 +38,10 @@
       [ ]
       [ null ]
     ];
+    duplicateChildren._children = [
+      { child = [ 2 ]; }
+      { child = [ 1 ]; }
+    ];
     extraAttrs = {
       _args = [
         2
@@ -52,6 +56,15 @@
         b = null;
       };
     };
+    argsAndProps = {
+      _args = [
+        1
+        2
+      ];
+      _props = {
+        a = 3;
+      };
+    };
     listInAttrsInList = {
       list1 = [
         { a = 1; }