Some refactoring I ran into while working on automatic Markdown list
continuation on newline.

This PR:
- Moves `comment_delimiter` and `documentation_delimiter` computation
outside of newline method.
- Adds `NewlineFormatting`, which holds info about how newlines affect
indentation and other formatting we need.
- Moves newline-specific methods into the new `NewlineFormatting`
struct.

Release Notes:

- N/A