JavaScript: Fix characteristic predicate of XMLParent.

The database type `@xmlparent` is defined a bit too loosely in that it includes all of `@file`, not just XML files. Fixing that would involve fiddling with the extractor/dbscheme, so I have opted to fix it at the QL level instead.

javascript/ql/src/semmle/javascript/XML.qll

@@ -34,6 +34,12 @@ abstract class XMLLocatable extends @xmllocatable {
  * both of which can contain other elements.
  */
 class XMLParent extends @xmlparent {
+  XMLParent() {
+    // explicitly restrict `this` to be either an `XMLElement` or an `XMLFile`;
+    // the type `@xmlparent` currently also includes non-XML files
+    this instanceof @xmlelement or xmlEncoding(this, _)
+  }
+
   /**
    * Gets a printable representation of this XML parent.
    * (Intended to be overridden in subclasses.)