Use Unicode process creation on Windows

[sheredom]

Summary

• switch Windows process creation from CreateProcessA to CreateProcessW
• convert command lines, custom environment blocks, and process CWD from UTF-8 to UTF-16 before spawning
• document Windows UTF-8 handling for command lines, environments, and CWD

Closes #68.

Tests

• cmake --build build && (cd build && ./subprocess_test)
• (cd build && ./subprocess_mt_test)
• cmake -S test -B /tmp/subprocess-clang2 -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DCMAKE_BUILD_TYPE=Debug && cmake --build /tmp/subprocess-clang2 && (cd /tmp/subprocess-clang2 && ./subprocess_test)
• (cd /tmp/subprocess-clang2 && ./subprocess_mt_test)

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