feat(linux): comments on module mcompile and filesystem 🐘

[SabineSIL]

fixes parts of PR https://github.com/keymanapp/keyman/pull/9384. Some comments may have been moved into an issue https://github.com/keymanapp/keyman/issues/11802

This PR addresses and bundles comments made on https://github.com/keymanapp/keyman/pull/9384 for the modules mcompile and filesystem

mcompile:

• use char*for filenames
• use char* instead of wchar_t*
• replace switch
• move run() and getDeadkeys()

filesystem:

• add else to if(isWin32Path)
• delete emscripten support
• check _MSC_VER

@keymanapp-test-bot skip

[keymanapp-test-bot[bot]]

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[ermshiperete]

Don't know why my reply doesn't show up here at the end as well... See above.