Convert Add File Banner refactoring to the new AbstractFileHeaderHelper analysis

[sharwell]

#41982 implements a new file header analysis helper AbstractFileHeaderHelper. The previous Add File Banner refactoring needs to be converted to this approach for consistency across multiple features.

1. Convert the current tests for Add File Banner to the new test library (which offers stronger guarantees and can better detect changes in behavior)
2. Convert Add File Banner to the new AbstractFileHeaderHelper, and address any changes in behavior

[CyrusNajmabadi]

We also have a few things that check banners i believe. like sort-usings (which doesn't want to move banners around, even though they're leading trivia), and potentially move/extract-type.

But i believe all these go through the common helpers. so if you fix those up, we should be good.

Note: these common helpers are on ISyntaxFacts. I'm 100% fine if you want to break them into their own special Helper class.