unified: get rid of undesirable self-recursion in _expression

This caused any field containing an _expression to appear as if it could
countain any number of such nodes. It also threw away the information
that there was a `?` marker there.

To fix it, we simply move the definition into its own named node.

Author: tausbn

unified/extractor/tree-sitter-swift/grammar.js

@@ -521,9 +521,11 @@ module.exports = grammar({
           $.assignment,
           $.value_parameter_pack,
           $.value_pack_expansion,
-          seq($._expression, alias($._immediate_quest, "?"))
+          $.optional_chain_marker
         )
       ),
+    optional_chain_marker: ($) =>
+      seq($._expression, alias($._immediate_quest, "?")),
     // Unary expressions
     _unary_expression: ($) =>
       choice(