feat(windows): multi-size .ico by default; deprecate windows.icon_size

[MetalHepple]

Summary

• Generate Windows .ico with multiple embedded sizes: 16, 24, 32, 48, 256.
• Ignore windows.icon_size; log a non-fatal deprecation notice if present.

Why

• Proper Windows icons need multiple sizes to avoid scaling artifacts.
• Sensible defaults simplify config.

Behavior

• Backwards compatible: existing pubspecs still parse (icon_size kept as deprecated).
• If windows.icon_size exists, we log a deprecation notice (no failure).

Tests

• Updated Windows validation test: warns instead of failing on icon_size.
• Fixed package_version_test.dart to find pubspec.yaml robustly across environments.
• dart analyze and dart test pass locally.

Migration

• Remove windows.icon_size from pubspec.yaml. No replacement needed.

Implementation Notes

• lib/windows/windows_icon_generator.dart: writes a multi-frame ICO (16, 24, 32, 48, 256) and logs deprecation.
• lib/config/windows_config.dart: keeps iconSize with @Deprecated for parsing only (ignored by generator).
• Tests updated accordingly; removed dead lint rule from analysis options.

Linked issues

Closes #573