feat(dune): support `+` prefixes for PPX CLI flags #29338

Job	Run time
Format	32s
Documentation	31s
Build microbenchmarks	31s
Create local opam switch (macos-latest, 5)	17s
Build monorepo benchmark docker image	28s
Create local opam switch (macos-latest, 4.14)	18s
Nix (macos-latest)	32s
Create local opam switch (ubuntu-latest, 5)	17s
Nix (ubuntu-latest)	31s
Build (macos-latest, 5.1.x)	1s
Wasm_of_ocaml	18s
Create local opam switch (ubuntu-latest, 4.14)	17s
Build (ubuntu-latest, 5.1.x)	17s
Build (ocaml-variants.5.3.0+trunk, ubuntu-latest, true)	16s
Coq 8.16.1	31s
Build (4.14.x, ubuntu-latest, true)	17s
Build (4.14.x, macos-latest, true)	19s
Build (5.1.x, macos-13, true)	1s
Build (ocaml-base-compiler.4.14.2,system-msvc, windows-latest)	14s
Build (ocaml-base-compiler.5.1.1,system-mingw, windows-latest)	11s
Build (4.08.x, ubuntu-latest, true)	17s
Build (4.04.x, ubuntu-latest, true, true)	17s
Build (4.02.x, ubuntu-latest, true)	17s
	7m 30s

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