Add an automated script to deploy Flutter demos.

[ilopX]

Add bin\deploy_flutter_demos.dart script.
This script will build a web platform flutter app and push the changes to web-demos branch on github.

Deploy flutter demos

1. Fork this repo: https://github.com/RefactoringGuru/design-patterns-dart
2. Apply your changes.
3. Run the script dart bin\deploy_flutter_demos.dart.
4. You can now make a pull request on the web-demos branch.
5. Once approved for the merge, the web app will be available at https://refactoringguru.github.io/design-patterns-dart .

Commands that this script executes:

create $tempDirectory
cd $tempDirectory
git clone -b web-demos --single-branch $frokOriginUrl .
git remote add upstream https://github.com/RefactoringGuru/design-patterns-dart.git
git fetch upstream
cd $projectDirectory
flutter build web -t bin\main.dart --web-renderer html
copyDirectory build\web\*.* $tempDirectory
cd $tempDirectory
git add .
git commit -m $lastProjectCommit
git push origin web-demos
delete $tempDirectory