Big internals refactoring

[avh4]

NOTE: If anyone's looking to contribute a PR, you should probably stay up-to-date with this branch, rather than current main.

This is some long-overdue refactoring:

• Clean up how the main code for the formatter is organized (and start to clearly separate Elm-specific logic from generic text layout logic)
• Separate code transformations (like removing unnecessary parens, fixing some invalid syntax, etc) from the formatting code. It will now be a separate pass that will be performed first.