Refactors DefautlTagHelperDescriptorFactory for clarity and efficiency

- Enable nullable
- Don't box ImmutableArrays to IReadOnlyLists
- Remove unused Compilation constructor parameter
- Expand and clarify logic for testing whether a property is "accessible".
- Avoid repeated LINQ expressions
- General code clean up and simplification
- Add a few helper extension methods to remove boilerplate.

Author: DustinCampbell