OCaml 5.4 support

[kit-ty-kate]

The freeze period for the OCaml 5.4 release cycle is planned for Friday and there should still be some changes in the parsetree between now and then so don't start now, but i'm opening this ticket in prevision for after Friday.

Currently ocamlformat fails to build with the following error:

#=== ERROR while compiling ocamlformat.0.27.0 =================================#
# context     2.4.0~alpha1~dev | linux/arm64 | ocaml-variants.5.4.0+trunk | https://opam.ocaml.org#34a3fc46c301a891122df75c17fef6b5358bd68f
# path        ~/.opam/5.4/.opam-switch/build/ocamlformat.0.27.0
# command     ~/.opam/opam-init/hooks/sandbox.sh build dune build -p ocamlformat -j 9 @install
# exit-code   1
# env-file    ~/.opam/log/ocamlformat-219150-0960e2.env
# output-file ~/.opam/log/ocamlformat-219150-0960e2.out
### output ###
# [...]
# 528 |       ; out_newline= (fun () -> ())
# 529 |       ; out_spaces= (fun _ -> ())
# 530 |       ; out_indent= (fun _ -> ()) }..
# Error: Some record fields are undefined: out_width
# (cd _build/default && /home/kit_ty_kate/.opam/5.4/bin/ocamlopt.opt -w -40 -noassert -open Ocamlformat_ocaml_common -open Ocamlformat_stdlib -g -I lib/bin_conf/.bin_conf.objs/byte -I lib/bin_conf/.bin_conf.objs/native -I /home/kit_ty_kate/.opam/5.4/lib/astring -I /home/kit_ty_kate/.opam/5.4/lib/base -I /home/kit_ty_kate/.opam/5.4/lib/base/base_internalhash_types -I /home/kit_ty_kate/.opam/5.4/[...]
# File "lib/bin_conf/Bin_conf.ml", lines 526-530, characters 6-35:
# 526 | ......{ out_string= (fun _ _ _ -> ())
# 527 |       ; out_flush= (fun () -> ())
# 528 |       ; out_newline= (fun () -> ())
# 529 |       ; out_spaces= (fun _ -> ())
# 530 |       ; out_indent= (fun _ -> ()) }..
# Error: Some record fields are undefined: out_width

[Julow]

I've fixed the build issue in #2717 but note that OCamlformat cannot format 5.4 code yet. The issues that need to be fixed before a release are listed in this milestone: https://github.com/ocaml-ppx/ocamlformat/milestone/18