Block assignment filters do not work

[dhruvkb]

Jinja supports block assignment since v2.8 and block assignment filters since v2.10. The latter is missing from Nunjucks and should be supported.

For example, the template below

{% set content | indent(2, true) %}
This is some text

and this is some text
{% endset %}
{{ content }}

when rendering, raises the error

Uncaught:
Template render error: (unknown path) [Line 1, Column 4]
  parseSet: expected = or block end in set tag