Support Flutter 3.44+ (IconData is now a final class)

[hiake21c]

Problem

Flutter 3.44 marked IconData a final class (flutter/flutter#181342). The package's core type does:

class PhosphorIconData extends IconData { ... }

Extending a final class is now a compile error, so every build on Flutter 3.44+ fails:

lib/src/phosphor_icon_data.dart:5:32: Error: The class 'IconData' can't be
extended outside of its library because it's a final class.

Fix

Icons become plain IconData instances instead of an IconData subclass. The generated style classes now emit IconData(...) directly with the identical codePoint / fontFamily / fontPackage, so flat icons render unchanged. PhosphorIconData, PhosphorFlatIconData and PhosphorDuotoneIconData are kept as type aliases for backward compatibility. SDK floor bumped to Dart 3.0 (non-function typedefs) / Flutter 3.44.

Tradeoff (needs a maintainer decision)

Because a plain IconData cannot carry a second layer, duotone icons collapse to their foreground layer (single-tone) in this PR. Preserving true two-tone rendering is possible but requires a breaking PhosphorIcon redesign (duotone can no longer be an IconData, so Icon(duotoneConst) and the PhosphorIcon extends Icon relationship both change). I kept this PR minimal to unblock 3.44 builds — happy to extend it to a duotone-preserving design if you prefer that direction.

Notes

• Constants were rewritten mechanically (the two generator output patterns), not hand-edited.
• Verified: a full app on Flutter 3.44.8 analyzes clean and builds against this branch.