fix(windows): plugin builds against current Flutter Windows registrant

[TheMystery28]

Summary

Two patches that together make ratex_flutter's Windows plugin build out-of-the-box against current Flutter (verified against Flutter 3.41.9 stable / Dart 3.11.5 on Windows 10).

Bug 1 — plugin's own .cpp can't find its own header

windows/CMakeLists.txt declares target_include_directories(... INTERFACE ...). INTERFACE exposes the dir only to consumers — the plugin's own ratex_flutter_plugin.cpp cannot resolve #include "ratex_flutter/ratex_flutter_plugin.h". Fix: widen scope to PUBLIC (strict CMake superset; consumers see no behavioural change).

Bug 2 — missing C API registrant

Flutter's auto-generated generated_plugin_registrant.cc (since Flutter ~2.10) includes <ratex_flutter/ratex_flutter_plugin_c_api.h> and calls RatexFlutterPluginCApiRegisterWithRegistrar(...). The plugin only shipped the legacy RatexFlutterPluginRegisterWithRegistrar. Fix: add the C API header + a one-line shim that delegates to the legacy entry, so both name variants resolve to the same body.

Reproduction

flutter create --platforms=windows test_app && cd test_app
flutter pub add ratex_flutter
flutter build windows --debug

Before this PR fails with:

ratex_flutter_plugin.cpp(7,10): error C1083: Cannot open include file:
  'ratex_flutter/ratex_flutter_plugin.h': No such file or directory

After commit 1 alone, build progresses further then fails at:

generated_plugin_registrant.cc(10,10): error C1083: Cannot open include
  file: 'ratex_flutter/ratex_flutter_plugin_c_api.h': No such file or directory

After commit 2 the build succeeds.

Backwards compatibility

• INTERFACE → PUBLIC: zero impact on consumers (PUBLIC is a strict CMake superset).
• Legacy RatexFlutterPluginRegisterWithRegistrar entry remains in ratex_flutter_plugin.cpp unchanged. Any consumer still using the legacy name (older Flutter, manual registrant) sees no change — the new entry just calls into it.

Test plan

• flutter build windows --debug succeeds on a fresh flutter create consumer (Flutter 3.41.9 stable / Win10)
• flutter build apk --debug still succeeds (Android path untouched)
• flutter build linux --debug not regressed (Linux CMakeLists untouched)
• iOS / macOS — please verify in CI (those platform dirs untouched)

Versioning / CHANGELOG

Left to the maintainer's release process. No version bump or CHANGELOG entry in this PR.

[erweixin]

thanks!