Consider C++20 module support

[wdhongtw]

Should we support the usage of this library as a C++ module? e.g. import OpenCC.

C++ module is a promising way to consume a library during compile time, comparing to header includes.
And there is a trend to support this since last year [1].
It would be good for OpenCC to provide module support.

Here is a working PoC at experiment branch, tested with MSVC and Clang(19.1):
https://github.com/wdhongtw/OpenCC/commits/feat-cxx-module/ .
It works in a ABI-compatible way [2], and just require an additional module file.

I wonder if it's a good timing to begin this discussion?

See also:

• [1] https://arewemodulesyet.org/
• [2] https://clang.llvm.org/docs/StandardCPlusPlusModules.html#transitioning-to-modules