doc.flex_break

[giacomocavalieri]

This PR adds flex_break to get flexible layouts that can adapt nicely to the maximum line width:

["1", "2", "3", "4", "5", "6"]
|> list.map(doc.from_string)
|> doc.join(with: doc.flex_break("", ""))
|> doc.group
|> doc.to_string(4)
|> should.equal("1234\n56")

Here the group is no longer split in an "all or nothing" way: only the flex breaks are split when necessary.

This is still a WIP since I need to:

• add more tests to see that flex breaks actually work
• add documentation with examples
• update the CHANGELOG
• make a new release
• (optionally) write another tutorial to cover this feature

[giacomocavalieri]

This PR is already way bigger than I anticipated so I'm merging it and I'll complete the tutorial in another one :)