Add CSS class support for task lists to resolve bullet point display issue

[Copilot]

This PR addresses the issue where task lists display both bullets and checkboxes, creating visual clutter. The solution adds automatic CSS class assignment to task lists, allowing developers to style them appropriately.

Problem

When creating task lists with checkboxes, browsers display both bullets and checkboxes:

- [x] Completed task
- [ ] Pending task

Previously rendered as:

<ul>
  <li><input type="checkbox" disabled checked />Completed task</li>
  <li><input type="checkbox" disabled />Pending task</li>
</ul>

This results in both bullets (•) and checkboxes (☑) being displayed, which is visually redundant.

Solution

The implementation automatically detects task lists and adds a configurable CSS class:

<ul class="task-list">
  <li><input type="checkbox" disabled checked />Completed task</li>
  <li><input type="checkbox" disabled />Pending task</li>
</ul>

Developers can now hide bullets with CSS:

.task-list {
  list-style: none;
}

Key Features

• Automatic detection: Task lists are identified by the presence of [x] or [ ] syntax
• Configurable CSS class: Default is task-list, customizable via lists.tasks.class
• Selective application: Only task lists get the class; regular lists remain unchanged
• Mixed list support: Lists containing both task items and regular items get the class
• Works with both <ul> and <ol>: Ordered task lists are also supported

Configuration

$parser = new ParsedownExtended();

// Default usage (class="task-list")
echo $parser->text("- [x] Done\n- [ ] Todo");

// Custom class
$parser->config()->set('lists.tasks.class', 'my-custom-task-list');

// Disable feature
$parser->config()->set('lists.tasks.enabled', false);

Breaking Changes

⚠️ Configuration structure updated:

• Old: lists.tasks (boolean)
• New: lists.tasks.enabled (boolean) and lists.tasks.class (string)

This change enables the new CSS class functionality while maintaining the ability to enable/disable task lists.

Fixes #43.

Warning

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If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to my firewall allow list

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