fix: Preserve block tags to preserve reconstruction/execution order

[jasmith-hs]

Simpler and more accurate fix to the bug that #1307 targeted (in that this also handles deferred modifications).

There is a bug in how reconstructed output is rendered when there are block tags due to the order of execution. Block tags are always evaluated last, even if they'll show up higher in a template

(I use {# \d+ #} in the following examples to show the order in which the lines are evaluated)

Child

{# 1 #}{% extends parent %}
{# 2 #}{% block foo %}
{# 6 #}{{ foo_bar }}
{# 2 #}{% endblock %}

Parent

{# 3 #}{{ 'Hello' }}
{# 4 #}{% block foo %}{% endblock %}
{# 5 #}{% set foo_bar = deferred %}

If reconstructed directly, we'd end up with:

{# 1 #}{{ foo_bar }}
{# 2 #}{% set foo_bar = deferred %}

Which results in foo_bar not being reconstructed in time. However, if we preserve the blocks, we can also preserve the execution order:

{# 1 #}{% block foo %}
{# 3 #}{{ foo_bar }}
{# 1 #}{% endblock %}
{# 2 #}{% set foo_bar = deferred %}