Unified: Add unhandled_node node type

This node type will be used to represent nodes that -- during
translation -- manage to reach the catch-all rule at the end. This
almost certainly indicates something went wrong.

In contrast, `unsupported_node` will now be used for nodes that we're
aware of, but which have not proper translation just yet.

Author: tausbn

unified/extractor/ast_types.yml

@@ -34,6 +34,7 @@ supertypes:
     - switch_expr
     - unresolved_operator_sequence
     - unsupported_node
+    - unhandled_node
     - or_pattern
     - conditional_pattern
     - bulk_importing_pattern
@@ -84,6 +85,7 @@ supertypes:
     - type_alias_declaration
     - associated_type_declaration
     - unsupported_node
+    - unhandled_node
   type_constraint:
     - equality_type_constraint
     - bound_type_constraint
@@ -421,6 +423,9 @@ named:
   # A node that we don't yet translate
   unsupported_node:
 
+  # A node kind that has no explicit translation or unsupported classification
+  unhandled_node:
+
   infix_operator:
 
   prefix_operator: