[WIP] Replace AST with CST

[gpetiot]

Please don't merge this!
The purpose is to visualize the diff between the current state of the ast+parser of ocamlformat and the cst+parser of ocamlformat-ng prototype (https://github.com/tarides/ocamlformat-ng), the files in this branch have been made closer to the current main branch of ocamlformat, but this doesn't build.

To display the diff: git diff main cst -- vendor/parser-extended/

This is a suggestion of roadmap (non-exhaustive) of the changes left to report in ocamlformat, it should be easier to start with the type changes before working on the ext_attributes and the parentheses. Refer to parsetree.mli:

1. Enrich existing type constructors

• ptyp_alias (Add location to ptyp_alias #2239)
• pexp_pack (Add optional package_type to pexp_pack #2234)
• pval_prim (Add locations to pval_prim strings #2238)
• pcstr_record (Add location to pcstr_record #2237)
• pcsig_self (Make pcsig_self optionnal #2216)
• pcstr_self (Make pcstr_self optional #2236)
• new type fun_param (Define type function_param and replace Sugar.arg_kind #2466)

2. Improve representation of poly/newtype

• redesign the whole representation of pexp_poly, pexp_newtype, ptyp_newtype_poly (Simplify representation of class field values and methods #2529)

3. Add location in Longident.t type

• replace occurrences ofLongident.t. loc with Longident.t

4. Add extension and attributes

• add fields *_ext_attributes (Moves module attributes after the module keyword #2247), as these fields are kept together in the parser.

5. Preserve parentheses

• Pexp_parens
• Ppat_parens
• Ptyp_parens
• Pcl_parens
• Pmty_parens
• Pmod_parens

Bonus

• types module_expr and module_type probably need to be reworked too
• undo the work made in Sugar.ml by reverting the normalization made in the parser

While reporting some of the changes into ocamlformat, if design choices make the files diverge from ocamlformat-ng please update the files in this branch.
Also don't forget to propagate the changes in parser-recovery/lib/parser.mly and run dune build @all to regenerate the diffs.

[Julow]

I've started working on the poly/newtype/annotation/coercion on let bindings (on this branch: https://github.com/Julow/ocamlformat/tree/concrete_let_bindings), which I'm putting on hold until #2401 is backported as it is largely conflicting.