Add a top-level tsconfig.json

The reason to add this is that I am getting misleadings errors in
vscode that this file is missing. By adding this file, I no longer
see these errors.

Author: aeisenberg

tsconfig.json

@@ -0,0 +1,3 @@
+{
+  "extends": "./extensions/ql-vscode/tsconfig.json"
+}