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 odoc fails to build with the following error:

#=== ERROR while compiling odoc.3.0.0 =========================================#
# 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/odoc.3.0.0
# command     ~/.opam/opam-init/hooks/sandbox.sh build dune build -p odoc -j 9 @install
# exit-code   1
# env-file    ~/.opam/log/odoc-219150-a3b893.env
# output-file ~/.opam/log/odoc-219150-a3b893.out
### output ###
# [...]
# (cd _build/default && /home/kit_ty_kate/.opam/5.4/bin/ocamlc.opt -w -40 -g -w -18-53 -g -bin-annot -bin-annot-occurrences -I src/loader/.odoc_loader.objs/byte -I /home/kit_ty_kate/.opam/5.4/lib/astring -I /home/kit_ty_kate/.opam/5.4/lib/camlp-streams -I /home/kit_ty_kate/.opam/5.4/lib/fpath -I /home/kit_ty_kate/.opam/5.4/lib/ocaml/compiler-libs -I /home/kit_ty_kate/.opam/5.4/lib/odoc-parser -[...]
# File "src/loader/cmti.ml", line 142, characters 20-29:
# Error: This record pattern is expected to have type Typedtree.package_type
#        There is no field pack_path within type Typedtree.package_type
# (cd _build/default && /home/kit_ty_kate/.opam/5.4/bin/ocamlc.opt -w -40 -g -w -18-53 -g -bin-annot -bin-annot-occurrences -I src/loader/.odoc_loader.objs/byte -I /home/kit_ty_kate/.opam/5.4/lib/astring -I /home/kit_ty_kate/.opam/5.4/lib/camlp-streams -I /home/kit_ty_kate/.opam/5.4/lib/fpath -I /home/kit_ty_kate/.opam/5.4/lib/ocaml/compiler-libs -I /home/kit_ty_kate/.opam/5.4/lib/odoc-parser -[...]
# File "src/loader/cmt.ml", line 56, characters 6-30:
# Error: The constructor Tpat_alias expects 5 argument(s),
#        but is applied here to 4 argument(s)
# (cd _build/default && /home/kit_ty_kate/.opam/5.4/bin/ocamlopt.opt -w -40 -g -w -18-53 -g -I src/loader/.odoc_loader.objs/byte -I src/loader/.odoc_loader.objs/native -I /home/kit_ty_kate/.opam/5.4/lib/astring -I /home/kit_ty_kate/.opam/5.4/lib/camlp-streams -I /home/kit_ty_kate/.opam/5.4/lib/fpath -I /home/kit_ty_kate/.opam/5.4/lib/ocaml/compiler-libs -I /home/kit_ty_kate/.opam/5.4/lib/odoc-p[...]
# File "src/loader/ident_env.cppo.ml", line 312, characters 4-31:
# Error: The constructor Tpat_alias expects 5 argument(s),
#        but is applied here to 4 argument(s)