Improve deferred word filtering

[jasmith-hs]

So that deferred words that aren't in the combined scope aren't attempted to be reconstructed in the wrong scope

Follow up on #1032

This test demonstrates the problem:

{% set my_list = [] %}
{% set foo %}
{% do my_list.append(deferred) %}a
{% endset %}
{{ my_list }}

We don't want my_list to get reconstructed inside the set tag so we should filter the deferred words that we reconstruct before deferring them to only reconstruct those within the current scope.

[jasmith-hs]

This line is the change, the rest is refactoring duplicated code