Generates Merlin config for executables using `future_syntax` preprocessing.

  $ ocamlc_where="$(ocamlc -where)"
  $ export BUILD_PATH_PREFIX_MAP="/OCAMLC_WHERE=$ocamlc_where:$BUILD_PATH_PREFIX_MAP"
  $ ocamlfind_libs="$(ocamlfind printconf path | while read line; do printf lib=${line}:; done)"
  $ export BUILD_PATH_PREFIX_MAP="$ocamlfind_libs:$BUILD_PATH_PREFIX_MAP"

  $ make_dune_project 3.11

  $ cat >dune <<EOF
  > (executable
  >  (name pp_future_syntax)
  >  (preprocess future_syntax))
  > EOF

  $ touch pp_future_syntax.ml

  $ dune build ./.merlin-conf/exe-pp_future_syntax --profile release
  $ dune ocaml merlin dump-config --format=json . | \
  >   jq -r '
  >     include "dune";
  >     merlinEntry("Pp_future_syntax")
  >     | (merlinPathLine | @json),
  >       (merlinConfigItems | @json)
  >   '
  "Pp_future_syntax: _build/default/pp_future_syntax"
  ["INDEX","$TESTCASE_ROOT/_build/default/.pp_future_syntax.eobjs/cctx.ocaml-index"]
  ["STDLIB","/OCAMLC_WHERE"]
  ["SOURCE_ROOT","$TESTCASE_ROOT"]
  ["EXCLUDE_QUERY_DIR"]
  ["B","$TESTCASE_ROOT/_build/default/.pp_future_syntax.eobjs/byte"]
  ["S","$TESTCASE_ROOT"]
  ["FLG",["-w","-40","-g"]]
  ["UNIT_NAME","dune__exe__Pp_future_syntax"]
  "Pp_future_syntax: _build/default/pp_future_syntax.ml"
  ["INDEX","$TESTCASE_ROOT/_build/default/.pp_future_syntax.eobjs/cctx.ocaml-index"]
  ["STDLIB","/OCAMLC_WHERE"]
  ["SOURCE_ROOT","$TESTCASE_ROOT"]
  ["EXCLUDE_QUERY_DIR"]
  ["B","$TESTCASE_ROOT/_build/default/.pp_future_syntax.eobjs/byte"]
  ["S","$TESTCASE_ROOT"]
  ["FLG",["-w","-40","-g"]]
  ["UNIT_NAME","dune__exe__Pp_future_syntax"]
  "Pp_future_syntax: _build/default/pp_future_syntax.mli"
  ["INDEX","$TESTCASE_ROOT/_build/default/.pp_future_syntax.eobjs/cctx.ocaml-index"]
  ["STDLIB","/OCAMLC_WHERE"]
  ["SOURCE_ROOT","$TESTCASE_ROOT"]
  ["EXCLUDE_QUERY_DIR"]
  ["B","$TESTCASE_ROOT/_build/default/.pp_future_syntax.eobjs/byte"]
  ["S","$TESTCASE_ROOT"]
  ["FLG",["-w","-40","-g"]]
  ["UNIT_NAME","dune__exe__Pp_future_syntax"]
