Add support for # inline comment tag

[Copilot]

This PR adds support for the # inline comment tag, as introduced in Shopify's Liquid PR #1498. This provides a simpler syntax for adding single-line comments without requiring an end tag.

New Syntax

{% # This is an inline comment %}

The inline comment tag supports whitespace trimming like other Liquid tags:

{%- # trimmed comment -%}

Examples

Basic usage:

Hello {% # this is a comment %} World

Output: Hello World

With whitespace trimming:

Hello{%- # comment -%}World

Output: HelloWorld

In templates:

{% # Start of section %}
{% assign name = 'John' %}
{% # Output the greeting %}
Hello {{ name }}!
{% # End of section %}

Output: Hello John!

Between control structures:

{% if true %}
  {% # This comment won't appear in output %}
  Success
{% endif %}

Output: Success

Implementation Details

• Reuses the existing CommentStatement AST node for consistency
• Modified tag parsing to handle # as a special character (not a valid identifier)
• Follows the same patterns as other inline tags like echo, break, and continue
• Supports whitespace trimming (-) markers like other Liquid tags

Testing

Added comprehensive test coverage:

• 4 parser tests to verify correct AST generation
• 6 template rendering tests to verify comments don't produce output
• All 1,201 tests pass with no regressions
• CodeQL security analysis passed with 0 alerts

Fixes the issue requesting support for the # comment tag as referenced in the Shopify Liquid enhancement.

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add support for # tag</issue_title>
<issue_description>> This PR adds a new tag named # that behaves like a comment.

Shopify/liquid#1498</issue_description>

<agent_instructions>Looked at the example in the referenced PR Shopify/liquid#1498
Add unit tests matching the comment tag.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@sebastienros It's not done, if you think you could give it a try that would be great. Personally, I am more focused on pre-compilation and breaking changes for the next major release.

Fixes #493

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[MihaxXx]

These changes allow # style comments in standard tags, e.g.:

{% # this is a comment %}

Remaining issue:

Inside {% liquid %} blocks, lines starting with # are still not treated as comments. Instead, the parser skips the following statements.

Example:

{% liquid
  assign name = 'John'
  # very important information
  assign name = 'Jenna'
  echo name
%}

Observed behavior:

assign name = 'Jenna' is skipped because the parser treats the # line incorrectly.

Expected behavior:

Lines starting with # inside {% liquid %} should be treated as comments. All other statements should execute normally.

Goal:
Extend the parser so that both {% # comment %} and inline # comments inside {% liquid %} blocks are fully supported, aligning with standard Liquid behavior.

The example from the initial description:

{% liquid
  # required args:
  assign product = product

  # optional args:
  assign should_show_border = should_show_border | default: true
  assign should_show_cursor = should_show_cursor | default: true
%}

@copilot can you take a look and prepare a PR to fix this problem?