Support Assignment Blocks with Set tags

[jasmith-hs]

For #693 and #422
Jinja 2.8 adds support for block set assignments (https://jinja.palletsprojects.com/en/3.0.x/templates/#block-assignments)

Add support for set tags with the notation:

{% set foo %}
A value of {{ 1 + 1 }}
{% endset %}

foo = 'A value of 2'

Additionally, Jinja 2.10 adds support for filters in the assignments:

{% set foo | upper %}
hello
{% endset %}

foo = 'HELLO'.

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The FlexibleTag interface adds a way to determine Node-based eligibility for requiring an EndTag rather than Tag-based eligibility through the hasEndTag(TagToken tagToken) method. Since this method takes a TagToken as the parameter, whether or not the tag will have a corresponding endtag depends on what is in the TagToken. For SetTag, it depends on whether there is an = in the helpers.

I'm opening this PR separately from the logic to handle this change with Eager Execution because those changes are much larger.

[jasmith-hs]

It is complex to properly support this with eager execution so it's in a separate PR #699

[jmaroeder]

Looks good, one question for my understanding!