Don't throw syntax errors in {% comment %} blocks

This is a bit of an opinion piece, but I don't think we should be throwing syntax errors for liquid in a comment block. A use case for comments is excluding code that you don't want to run—this means it may be incomplete and not working. I think this was already considered to a degree since there's an escape hatch for tags that aren't registered (via unknown_tag overwrite). For example:

This will successfully parse:

{% comment %}
  {% not_a_valid_tag %}
{% endcomment %}

This will throw a syntax error:

{% comment %}
  {% cycle %}
{% endcomment %}

I believe both of these should parse without error.

Start by examining how the parser handles {% comment %} blocks and how unknown_tag and cycle tags are validated. Reproduce the two Liquid examples from the issue, then verify that both incomplete and registered-but-invalid tags inside comments parse without syntax errors.