Improve Code Quality 2

[iamcarbon]

This PR:

• Reduces dictionary lookups
• Reduces allocations
• Update AngleSharp dependency
• Refactors a few things in preparation to enable nullable

[iamcarbon]

Ready for review.

[gabriele-tomassetti]

Thanks for the thorough review of the code.

I originally did try to mantain a syntax close to the original JavaScript to make easier to port any new change from upstream. The less change there is, the easier is to add updates. My concern was to avoid the common case of abandoned ports because of a mix of a lack of interest and too much effort required. I wanted to make sure to be able to keep the pace.

However, most people seemed to prefer a more C#-native style and we already heavily improved the overall organization of the code. So it make sense to also improve the syntax of the code. All this to say: thanks for your improvements to bring a more C#-native syntax to the library.