Refactor-ish tasks for version 3

[ultraq]

Some things to try for the next release, no guarantee I'll get around to them but it'll be neat to help clean up the codebase while I'm working on it:

• [ ] ~Add extension methods to traverse models as if they were DOM elements, namely the ability to get the 'first element child' like https://developer.mozilla.org/en-US/docs/Web/API/Element/firstElementChild~
• [ ] ~Convert the ChildModelIterator into a general purpose model iterator. In conjunction with the above, that can replace the need for a specific child model iterator.~
• [x] Find some way to better identify the model vs event extension methods in IModelExtensions: I got confused recently as it's not always clear which methods operate on what kind of objects
• [x] The whitespace creation in the insertModelWithWhitespace extension method could do with some work - it inserts Windows line ends instead of platform-specific ones, and its opposite removeModel doesn't have a "with whitespace" version which could clear up some formatting issues in the resulting output

[ultraq]

Worked on what I could, but wow whitespace handling is a little annoying when you have to do it yourself! As for the other stuff, it's also kind of in the difficult basket no wonder it's been kinda ad-hoc all these years! 😅