Formatter fix: next_break_fits respects line_length

[sabiwara]

Tentative fix for #13775

The problem is that the closing bracket ")" is ignored when evaluating the nested group for which fitting is enabled.

["f" · disable([
  ²⟨"(" · _⟩ · ²⟨enable([
    ²⟨"%" · ∅ · "{" · _ · ["x"]⟩ · _ · "}"
  ])⟩ · _ · ")" · ∅
])]

The approach:

• fits? doesn't return a bool anymore, but :fit | :no_fit | :break_next, :break_next meaning that we wouldn't actually fit without breaking within the nested :enabled
• add more modes to format to make sure we break in the nested :enabled

[josevalim]

@sabiwara I need to dig deeper but can we break this PR or this commit in two? One that goes from boolean to :fit | :no_fit and another one with the fix? This will make it easier to assess the impact of the change. :)

[sabiwara]

can we break this PR or this commit in two? One that goes from boolean to :fit | :no_fit and another one with the fix?

Of course! Sorry the diff wasn't very readable indeed.

I need to dig deeper

Yes this is a tricky one and my confidence is not great, I'm just starting to understand how the whole fitting works.

[sabiwara]

@josevalim should be much better 😌 d40b032

[josevalim]

This looks great to me! Thank you for working on this. ❤️