Fix is_of_type behavior in output filters

For the following top-level input param:

```

```

and these outputs:

```

unqs.is_of_type("A") or unqs.is_of_type("B")

unqs.is_of_type("C")

```

always both outputs are added.

In my A,B,C are data types derived from Tabular in my case.

@mvdbeek suggested to filter with `unqs.ext == 'A'` which works. But using the is_of_type function would be nicer.

Reproduce the issue with the shown top-level data parameter and output filters, then trace how output filters evaluate is_of_type versus unqs.ext. The fix is done when the A/B filter adds only stable_uniques and the C filter adds only stable_mergepairs, while the existing ext comparison continues to work.

Written by the indexing model from the issue text.