chore(ast-generic): Add Cls variant (#329)

- [X] I ran `make setup && make` to update the generated code after
editing a `.atd` file (TODO: have a CI check)
- [X] I made sure we're still backward compatible with old versions of
the CLI.
For example, the Semgrep backend need to still be able to *consume* data
	  generated by Semgrep 1.50.0.
See
https://atd.readthedocs.io/en/latest/atdgen-tutorial.html#smooth-protocol-upgrades
	  Note that the types related to the semgrep-core JSON output or the
	  semgrep-core RPC do not need to be backward compatible!

ast_generic_v1.atd

@@ -287,6 +287,7 @@ type field_name = [
 type special = [
   (* special vars *)
   | This | Super (* called 'base' in C# *)
+  | Cls (* similar to `self`, but indicating the type of the class, not the instance *)
   | Self | Parent (* different from This/Super? *)
 
   (* special calls *)

ast_generic_v1_j.ml

@@ -1361,6 +1361,7 @@ let write_special = (
     match x with
       | `This -> Buffer.add_string ob "\"This\""
       | `Super -> Buffer.add_string ob "\"Super\""
+      | `Cls -> Buffer.add_string ob "\"Cls\""
       | `Self -> Buffer.add_string ob "\"Self\""
       | `Parent -> Buffer.add_string ob "\"Parent\""
       | `Eval -> Buffer.add_string ob "\"Eval\""
@@ -1435,6 +1436,10 @@ let read_special = (
               Yojson.Safe.read_space p lb;
               Yojson.Safe.read_gt p lb;
               `Super
+            | "Cls" ->
+              Yojson.Safe.read_space p lb;
+              Yojson.Safe.read_gt p lb;
+              `Cls
             | "Self" ->
               Yojson.Safe.read_space p lb;
               Yojson.Safe.read_gt p lb;
@@ -1578,6 +1583,8 @@ let read_special = (
               `This
             | "Super" ->
               `Super
+            | "Cls" ->
+              `Cls
             | "Self" ->
               `Self
             | "Parent" ->