[WIP] Add new formatter that can do better pretty-printing

[joshuawarner32]

Instead of trying to do formatting all in one go, instead build up an efficient tree structure first and only after that decide where to place newlines/etc.
Notably, this let's us use relatively simple/clean heuristics for removing newlines as long as the overal visual line length remains manageable.
It also removes the need for separate is_multiline/format code, where the former has to infer what the latter will do.

Highly WIP! Not ready for merging!

[kdziamura]

Related issue: #6903

[github-actions]

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