[flutter_plugin_tool] Add support for building UWP plugins (#4047)

This allows building UWP plugin examples with `build-examples --winuwp`. As with previous pre-stable-template desktop support, this avoids the issue of unstable app templates by running `flutter create` on the fly before trying to build, so a template that will bitrot doesn't need to be checked in.

Also adds no-op "support" for `drive-examples --winuwp`, with warnings about it not doing anything. This is to handle the fact that the LUCI recipe is shared between Win32 and UWP, and didn't conditionalize `drive`. Rather than change that, then change it back later, this just adds the no-op support now (since changing the tooling is much easier than changing LUCI recipes currently).

This required some supporting tool changes:
- Adds the ability to check for the new platform variants in a pubspec
- Adds the ability to write test pubspecs that include variants, for testing

Part of https://github.com/flutter/flutter/issues/82817

script/tool/test/create_all_plugins_app_command_test.dart

@@ -21,7 +21,7 @@ void main() {
     setUp(() {
       // Since the core of this command is a call to 'flutter create', the test
       // has to use the real filesystem. Put everything possible in a unique
-      // temporary to minimize affect on the host system.
+      // temporary to minimize effect on the host system.
       fileSystem = const LocalFileSystem();
       testRoot = fileSystem.systemTempDirectory.createTempSync();
       packagesDir = testRoot.childDirectory('packages');