Updated Target Frameworks to: netstandard2.0, net 4.6.2, net 6, net 7

[SpaceOgre]

The following is done in this PR

Fixes: #6

• Updated the target frameworks to:
  • netstandard 2.0
  • net4.6.2
  • net6.0
  • net7.0
• Changed testing framework to xUnit (this is the recommended framework by Microsoft these days)
  • Added some more test cases
• Added support for DefaultExpression
• Cleaned the code in the Comparer class
  • Using pattern matching
  • Adding else to the if-statemensts since only one can be true
  • Using Lazy-evaluation for IsAnonymousType
  • TryCalculateConstant is only done for y if the call to TryCalculateConstant(x) returns IsDefined == true
• Added an .editorconfig file based on the one used by Microsoft for .Net Core

[SpaceOgre]

If you like this PR I think it would be best to merge it with a Squash, the commit history is not that nice :)

[nullptre]

Thanks @SpaceOgre, I didn't check it yet though, and I'm planning to dedicate some time this weekend for the PR. Pls let me know how long you can wait.

[SpaceOgre]

@yuriy-nelipovich it is your repository 😉
I dont need it yet for a time so no hurry from me.