Added Workflow so it tests the code on each push/PR to master

[behzodfaiziev]

Initial issue:

• One of my previous commits made the Vexana score to fall to 140 points due to a minor formatting issue :(
  

Possible solution:

• in order to resolve the problem, it is required to add 1 space to get 150 points again.
  

But this issue may be reproduced again and again.

Better solution: (Learnt from @VB10 that there always should be a thoughtful action)

• in order to improve the overall quality of the Vexana, I decided to completely automatize the: formatting, analyze and test. So I added a workflow which would be triggered on each PR and push to master. Thus it would make sure that any code which is not of high quality would not pass the overall test.

Additional steps to make tests pass

• added sort_pub_dependencies, so it does not show error of sorting dependencies
  

• added void main(){} in network_manager_test.dart because if the test file is empty, the test will automatically fail.
  

[behzodfaiziev]

@VB10 also when someone opens PR, it would automatically trigger the unit_test.yaml action so that you make sure that everything is fine before reviewing