Add support for C++ modules

[mikomikotaishi]

This pull request adds support for C++20 modules to SQLiteCpp, by creating modules sqlite.*. It is built using the macro SQLITECPP_BUILD_MODULES. The README.md file has also been updated to contain information.

I also added a namespace sqlite = SQLite; alias so that the namespace and module names align.

[mikomikotaishi]

I am wondering, could we also add a preprocessor macro to export the symbol Exception as SQLException (similar to its name in Java), so that it is not confused with the usual std::exception from the standard library?

[SRombauts]

This would be something for C++20, right now I am only considering making C++17 the baseline...
I'll have a second though at that in the following days

[mikomikotaishi]

It is an opt-in feature, and doesn't prevent C++17; it just makes the option available if the user compiles as C++20 and above, because the configuration is guarded in CMake.

[SRombauts]

Noted!
I am going to release a final version supporting C++11 to help overhaul the entire project, so this is something to keep in mind for right after