Do not use extension nodes for sugared extension forms

[gpetiot]

Fix #2081

• Preserves more of the sugared extension forms (wasn't the case when we formatted a function in an Apply when break-infix-before-func=false for example)
• Behaves the same with/without extension (for example regarding module-item-spacing (only diff we get when running test_branch.sh))
• Removes code duplication and the whole extension-node layer in Fmt_ast.ml and Ast.ml
• Removes Source.extension_using_sugar that relied on the input anyway