Windows support for package management

[gridbugs]

This issue collects all the issues that need to be addressed in order to support dune package management on windows:

• Allow users to choose a C compiler toolchain on windows #11160
• Support installing the compiler to the toolchains directory on windows #11159
• Install the "flexdll" package in the toolchains directory on windows #11158
• On windows, (run ./configure) results in CreateProcess(): Exec format error #11174
• Error fetching compiler package on Windows relating to directory targets #12429
• Tests fail on Windows with: Cannot decode build prefix map #10176
• Can't install ocamlfind as package on Windows due to missing .exe extension in ocamlfind.install #12430
• Can't build ocamlbuild on Windows: Error: opendir(): No such file or directory #12431
• Package build rules can't run ocamlfind's ocaml shell script on Windows #12432
• Dune package management on Windows cannot find the menhir executable even when it exists #12433
• Too easy to reach MAX_PATH on windows #11303
• Use hard links when sandboxing on Windows when possible #4362
• sandbox cleanup fails on Windows when junctions are present #13909
• unicode in sandbox prevents sandbox cleanup on Windows #13993

Additionally, we'll need to make some changes to the binary distribution to support windows:

• Build the binaries for windows ocaml-dune/binary-distribution#139
• Install script for windows ocaml-dune/binary-distribution#140
• Windows version of the ocamllsp wrapper script ocaml-dune/binary-distribution#141

We'll also need to start building the ocamllsp and ocamlformat packages for windows:

• Build ocamlformat and ocamllsp packages for windows ocaml-dune/ocaml-binary-packages#18

[ManasJayanth]

IMO, the first step in Windows support is deciding what toolchain we support. I think we could start with one (MSVC or Mingw) because the two toolchains bring a lot of diverging challenges. I left a comment here regarding a possible first step.

[gridbugs]

I recently got a couple of packages building on Windows with Dune package management (the packages were ocamlformat and ocaml-lsp-server). It required working around several issues. I've made this repo with simple reproduction cases for each of the problems I ran into.