PHPLIB-1597 Accept a $-prefixed string anywhere an expression is accepted

Author: GromNaN

generator/config/expressions.php

@@ -45,6 +45,11 @@
 $expressions = [];
 $resolvesToInterfaces = [];
 foreach ($bsonTypes as $name => $acceptedTypes) {
+    // an expression can be a string with a $-prefixed field name
+    if (! in_array('string', $acceptedTypes)) {
+        $acceptedTypes[] = 'string';
+    }
+
     $expressions[$name] = ['acceptedTypes' => $acceptedTypes];
 
     $resolvesTo = 'resolvesTo' . ucfirst($name);