Ensure that Scala 2.13 can transform Scala 3 code and vice versa

[MateuszKubuszok]

TODO:

• update build
  • test locally that artifacts are not broken
• define 2.13 and 3 test cases
  • case classes
  • case classes with @BeanProperty
  • classes with default values in constructor
  • enums with parameters in any value
  • enums without parameters in any value
  • sealed traits
• tests 2.13 converting 3 classes and vice versa
  • case classes
  • @BeanProperties as source/target
  • sealed traits to/from enums
  • default values in target type
• documentation - describe what is possible and what isn't

Updated and fixed version of #475

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.55%. Comparing base (7d29a86) to head (0c86ba2).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #647      +/-   ##
==========================================
+ Coverage   86.48%   86.55%   +0.06%     
==========================================
  Files         158      158              
  Lines        6225     6240      +15     
  Branches      559      539      -20     
==========================================
+ Hits         5384     5401      +17     
+ Misses        841      839       -2     

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