flutter_widget_from_html Add support for string literals in CSS list-style-type property

Fixes #1504

This PR implements support for CSS string literals as list-style-type values, allowing developers to use custom text markers for HTML lists.

What's Changed

Previously, the list-style-type property only supported predefined CSS keywords like disc, circle, decimal, etc. This implementation adds support for quoted string literals as specified in the CSS specification.

Examples

<!-- Custom numbered markers -->
<ul style="list-style-type: '① '">
  <li>First item</li>
  <li>Second item</li>
</ul>

<!-- Custom text markers -->
<ol style="list-style-type: 'Step '">
  <li>Initialize</li>
  <li>Process</li>
</ol>

<!-- Empty string (no marker) -->
<ul style="list-style-type: ''">
  <li>Item without marker</li>
</ul>

Implementation Details

String Detection: Modified getListMarkerText() to treat any unrecognized list-style-type value as a string literal
Quote Handling: The CSS parser already handles quote extraction and HTML entity decoding (" → ")
Empty Strings: Empty string literals properly render without markers (like list-style-type: none)
Compatibility: All existing functionality preserved - geometric markers (disc, circle, square) and serial markers (decimal, roman, alpha) work unchanged

Technical Changes

Updated getListMarkerText() in core_widget_factory.dart to return unrecognized types as-is
Modified buildListMarker() to handle empty string literals by returning null (no marker)
Added _isPredefinedListStyleType() helper to distinguish between CSS keywords and string literals
Enhanced test coverage with comprehensive string literal test cases