Enable Windows Emscripten build

[mcbarton]

Description

Please include a summary of changes, motivation and context for this PR.

This PR enable xeus-cpp to be cross compiled to Emscripten on a Windows platform (following Emscripten build instructions in CppInterOp). The 2 changes are as follows

1. Add .bat suffix to embuilder if on Windows (otherwise it uses the one made for Unix systems and errors)
2. Change \ to / in ESCAPED_XEUS_CPP_RESOURCE_DIR and ESCAPED_SYSROOT_PATH (\ gets removed from the preloaded path otherwise)

To see these changing allow an Emscripten build of xeus-cpp to work and pass all Emscripten tests, see this PR in CppInterOp compiler-research/CppInterOp#647

Fixes # (issue)

Type of change

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• Bug fix
• New feature
• Added/removed dependencies
• Required documentation updates

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.78%. Comparing base (3d2e16f) to head (5391dcd).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #362   +/-   ##
=======================================
  Coverage   81.78%   81.78%           
=======================================
  Files          20       20           
  Lines         950      950           
  Branches       87       87           
=======================================
  Hits          777      777           
  Misses        173      173           

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[vgvassilev]

Is this a better way to detect the if the compiler is running on windows: emscripten-core/emscripten#3191

[mcbarton]

This is the way we have been detecting we are doing an Emscripten build of Windows within CppInterOp. Its possible we might just be able to use EMCC_SUFFIX from the Emscripten.cmake file, which we use as part of the build. Its probably more reliable too, since it will update with whatever Emscripten decides the suffix should be. I will modify the cmake to use this, and then rerun the workflow in CppInterOp to check it works.

[vgvassilev]

Do we want to work in the windows ci here, too?

[mcbarton]

Do we want to work in the windows ci here, too?

@vgvassilev xeus-cpp uses Emscripten forges emsdk, which is only available for very particular platforms. When we asked previously for them to make emsdk available on Windows before the request was rejected. I could add a Windows Emscripten build here too, but we would need to get the upstream emsdk to achieve this. What would you like me to do?