This PR makes it so Fuzi can be used in Swift projects on Windows.

1. Add .build/ to .gitignore.
2. Add CMakeLists.txt where needed to build Fuzi with CMake.
3. Add Vendor with CMakeLists.txt to fetch and build LibXml2 on Windows, which is not available by default.
4. Fix issue relating to the fact that a C long on Windows is 4 bytes instead of 8.
5. CoreFoundation is not available on Windows, so replace usage of CFXxx() functions to map IANA encodings to String.Encoding with a String.Encoding extension that does the mapping.
6. Fix a problem with libxml2 C functions wanting a mutable pointer argument.
7. Fix issue where one member of a set of flags is a #define instead of a member of the enum proper so it is imported as the wrong type on Windows.

To build on Windows, use the Ninja generator:

cmake -S . -B build -G Ninja
cmake --build .\build\

Verified that the project still builds on MacOS with swift build.