Ignore generated folder in source database

When we run the tutorial query, we generate some files in a
new folder:

```
codeql-tutorial-database/db-csv/default/strings/
```

We don't want the user to see this in their codespace so let's
ignore it.

Author: elenatanasoiu

.gitignore

@@ -1,3 +1,5 @@
 # CodeQL metadata
 .tours/codeql-tutorial-database/db-csv/default/cache/
+.tours/codeql-tutorial-database/db-csv/default/strings/
 .cache/
+