Refactor Generators code

[dex3r]

Code under namespace EasySourceGenerators.Generators.IncrementalGenerators is currently a mess. Let's refactor it to be readable, maintanable and testable.

Besides general good coding rules, focus on:

• Don't store global state, pass necessary data as parameters
• Create small classes with small methods, that only do one thing
• make sure the classes are unit-testable
• if needed, create abstraction layer for Roslyn Generators and other parts. minimize using Roslyn Generators code to a small subset of classes that need them, making most of the code logic unit-testable without running any actuall generators
• extract all parts of code that emit code as strings (like "{" or "private") into separate namespace with separate classes. create unit tests for them.
• create documentation ///<summary> comments for internal/public methods when name is not enough