JS: Add initial file threat-model support

However, as indicated by the `MISSING` annotations, we could do better.

javascript/ql/lib/semmle/javascript/Concepts.qll

@@ -122,6 +122,19 @@ abstract class FileSystemReadAccess extends FileSystemAccess {
   abstract DataFlow::Node getADataNode();
 }
 
+/**
+ * A FileSystemReadAccess seen as a ThreatModelSource.
+ */
+private class FileSystemReadAccessAsThreatModelSource extends ThreatModelSource::Range {
+  FileSystemReadAccessAsThreatModelSource() {
+    this = any(FileSystemReadAccess access).getADataNode()
+  }
+
+  override string getThreatModel() { result = "file" }
+
+  override string getSourceType() { result = "FileSystemReadAccess" }
+}
+
 /**
  * A data flow node that writes data to the file system.
  */