Add support for nested block comments

[charlespwd]

Fixes #74

The language allows us to do this (for convenience). This PR adds fixes support for that.

The technique used is a bit similar to the one I used for Syntax Highlighting.

Before

A "comment" block was considered a "raw" tag block. We'd parse it like this:

• match /{%-?\s*comment\s*-?%}/
  • that's the .blockStart of the tag
• match all characters until one that is followed by /{%-?\s*endcomment\s*-?%}/
  • that's the body of the tag
• match /{%-?\s*endcomment\s*-?%}/
  • that's the .blockEnd of the tag

This approach didn't work for nested tags, since we'd match over the {% comment %} of the nested comment and stop at the {% endcomment %} of the nested comment. Leaving us with a dangling comment body and {% endcomment %}.

Something like this:

{% comment %}
This is inside the comment
  {% comment %}
    nested
  {% endcomment}
This is outside the comment since we stoped parsing on the previous line
{% endcomment %}

After

Slightly different algorithm. We parse like this:

• match /{%-?\s*comment\s*-?%}/
  • that's the .blockStart of the tag
• match as many of the following two things (the concatenated list is the body of the tag):
  1. Another comment block (recursive rule)
  2. all characters until one that is followed by one of those two:
     • /{%-?\s*comment\s*-?%}
     • /{%-?\s*endcomment\s*-?%}/
• match /{%-?\s*endcomment\s*-?%}
  • that's the .blockEnd of the tag

The "trick" here is that we stop parsing on nested {% comment %} and try to recurse on the rule.

So this will work:

{% comment %}
This is inside the comment
{% comment %}
  this is nested inside a recursed rule
{% endcomment %}
This is inside the top comment
{% endcomment %}