Replace golden_toolkit with alchemist

[DDavidPrime]

As a developer I want to write golden tests easily and without much though about the way how to implement the system. For that I want to use a package that is relatively simple to use and maintained by the community. Such choice has boiled down to the Alchemist package which should replace the discontinued golden_toolkit package.

ACs:

• Replace golden_toolkit with alchemist package (covered in Update rx_bloc_cli generated projects to use alchemist #844 )
  • Implement initial setup
  • Update existing tests
• Update example apps ( ETA: 1.5d)
  • Todo app Update golden tests with alchemist in Todo app #863
  • Reminders app Update golden tests with alchemist in Reminders app #864
  • Booking app Update golden tests with alchemist in Booking app #865
• Update documentation ( ETA: 2h)

Note:

• Updating tests in rx_bloc_cli generated projects will be handled in a separate ticket ( Update rx_bloc_cli generated projects to use alchemist #844 )
• Updating the IntelliJ plugin to support the new alchemist option will be dealt with in a separate ticket ( IntelliJ plugin: Add support for golden tests using alchemist package #846 )