Update flutter-add-widget-preview skill to match Flutter 3.47 Widget Previewer docs

[jesskuras]

Updates skills/flutter-add-widget-preview/SKILL.md to align with recent updates to the Flutter Widget Previewer documentation (flutter/website#13704 and flutter/website#13861):

• Updates the IDE version requirement to Flutter 3.47+ (reflecting Widget Previewer stability in Flutter 3.47).
• Updates the custom PreviewThemeData example to extend PreviewThemeData (@override Widget apply(BuildContext context, Widget child)) and call builder.build() instead of the removed PreviewThemeData(materialLight: ..., materialDark: ...) constructor.
• Expands the @Preview parameter reference (name, group, size, textScaleFactor, wrapper, theme, brightness, localizations).
• Adds guidance on preview search and filtering criteria (preview name, group name, containing script, and containing package), preview card controls, and .widget_preview/ build caching.

Note: This is AI-generated (I'm testing an automation to try to update skills based on doc updates). I looked at the suggestions and it seemed like a good update to make, but feel free to be harsh on it so I can improve it. Thanks!!

[gemini-code-assist]

Code Review

This pull request updates the Flutter Widget Previewer documentation to include a new section on preview parameters, details on build caching, updated IDE support requirements, and instructions for searching, filtering, and controls. It also revises the code examples to demonstrate custom previews with PreviewThemeData and runtime transformations. Feedback on these changes suggests improving text readability by removing repetitive phrasing, handling potential null values for preview names to avoid displaying 'null' in the UI, and defensively using MediaQuery.maybePlatformBrightnessOf to prevent runtime exceptions when a MediaQuery ancestor is missing.