add windows support for example

[smw-wagnerma]

added windows desktop support to the example, works fine. Only the rust dll must be copied to build/windows/runner/Debug, perhaps cargo can handle it and copy it the right location for each plattform.

[fzyzcjy]

Thank you so much! Just a few small questions:

• I wonder where the ~1000loc comes from - I guess firstly follow the Flutter's official Desktop tutorial, and then edit a bit of our .dart code?
• Could you please modify (or add) subsection in this section of readme, explaining what a user should do to run example in windows
• Could you please add a line in changelog
• Could you please add some tests in the GitHub CI in this file (just following the existing flutter_android_test/flutter_ios_test and add a flutter_windows_test) - this will ensure that it still works on windows in the future development of this lib, and also provides a command-by-command reference to anyone who wants more detail in reproducing it.

[Desdaemon]

Might be off topic, but I found a way to have CMake build Rust crates and automatically add them to the correct folders via corrosion. If there is interest I will make a minimum example.

[fzyzcjy]

#130 has been merged so I closed this. Feel free to reopen if this still needs to be merged!